It just bumps against the hard problem again — Harry Hindu

Because you keep asking the hard question. We don't have an answer to it.

All I am explaining is what the science shows; that pain and colour are percepts that occur when there is the appropriate brain activity; they are not mind-independent properties of knives and pens.

How does a colorless process create color? — Harry Hindu

How does a painless process create pain?

Any time you ask me a question like this about colour, just ask the same question about pain. Colour is just like pain, whatever pain is.

What is pain? — Harry Hindu

A percept that occurs when there is the appropriate neurological activity, often in response to electrical signals sent from nociceptors.

See for example Role of the Prefrontal Cortex in Pain Processing:

The main brain areas that are most consistently activated under painful conditions are the insular cortex and secondary somatosensory cortex, bilaterally. Electrical stimulation of these areas, but not in other candidate brain areas, is able to elicit a painful sensation.

In other words, it isn't known whether color experiences require the appropriate neurological activity..., In other words it is possible that colors ain't just in the head. — Harry Hindu

So in other words it isn't known whether pain requires the appropriate neurological activity, and so it is possible that pain just ain't in the head?

Maybe pain really is some mind-independent property of the knife that my body can sense when I'm stabbed with it.

What's so special about neurological activity that causes color? — Harry Hindu

What's so special about neurological activity that causes pain? This is the hard problem of consciousness that is yet to be solved.

It means that the colour ain't in the head. — jkop

No it doesn't. That colour experiences require neural connections ordinarily formed in response to electrical information from the eyes does not entail that colours are mind-independent properties of light or a material surface that reflects such light.

If that was true, then you could make the blind see by merely stimulating parts of their brains. — jkop

We're working on it.

See a narrative review of cortical visual prosthesis systems: the latest progress and significance of nanotechnology for the future.

But since their brains have never recieved the right stimulation (e.g. from the eyes via the optic nerve), then the right neural connections for colour-vision have not been developed,. — jkop

That may also be true, but does not refute anything I have said. It certainly does not entail that colours are mind-independent properties of pens.

If the pen has a surface layer of atoms that reflect light at with a wavelength of ~700nm then there is both red in the pen and the pen is red. — Leontiskos

Except when we say that the pen is red we are not (ordinarily) saying that the pen has a surface layer of atoms that reflects light with a wavelength of ~700nm.

e.g. when we explain variations in colour perception, such that some see a white and gold dress and some see a black and blue dress, we are not explaining that different people are seeing different objects reflecting different wavelengths of light. They are all looking at the same object reflecting the same wavelengths of light but see different colours. The colours they see, and that we are talking about, are not micro-structural properties or reflectances of the computer screen; the colours they see are mental percepts, whether they recognise them as colour percepts or not.

it looks as if you believe that there are mind-independent micro-structural properties that are not responsible for colour — Banno

Then you are not reading what I am writing. So I'll refer you back to the previous post that was directed at you:

The pen just has a surface layer of atoms that reflects light with a wavelength of ~700nm. When light stimulates the eyes it causes the neurological activity responsible for colour percepts, and we name the colour percept ordinarily caused by 700nm light "red".

Your claim that there is red "in" the pen is the naive realist view that science has disproven.

The "common sense" view, before any scientific study, is naive realism:

Naive realism
1. Colours, as ordinarily understood, are sui generis, simple, intrinsic, qualitative, non-relational, non-reducible properties
2. These sui generis properties are mind-independent.

This view contrasts with something like dispositionalism:

Dispositionalism
3. Colours, as ordinarily understood, are micro-structural properties or reflectances.
4. These micro-structural properties are mind-independent.

(1) and (4) are true, (2) and (3) are false.

The fact that people talk about redness as if it is mind-independent does not entail that they are talking about redness as if (3) is true. People tend to talk about redness as if both (1) and (2) are true. People don't tend to think about (3) at all. I suspect many people, especially children, wouldn't even understand (3); but they understand colours.

Those sui generis properties that we ordinarily think about when we think about colours are, in fact, mental phenomena, and not mind-independent properties of pens as some believe.

Neural representations of perceptual color experience in the human ventral visual pathway:

There is no color in light. Color is in the perceiver, not the physical stimulus. This distinction is critical for understanding neural representations, which must transition from a representation of a physical retinal image to a mental construct for what we see. Here, we dissociated the physical stimulus from the color seen by using an approach that causes changes in color without altering the light stimulus. We found a transition from a neural representation for retinal light stimulation, in early stages of the visual pathway (V1 and V2), to a representation corresponding to the color experienced at higher levels (V4 and VO1). The distinction between these two different neural representations advances our understanding of visual neural coding.

So colour experiences change when the neural activity in V4 and VO1 changes.

Say that a coloring agent is added to a clear pen in order to make it red. Different agents can be added to different pens in order to add different color to the plastic of the pen. Pigments and coloring agents exist out there, in the pen, independent of the mind. I can’t see the color anywhere else, whether beside it, in front of it, or somewhere behind my eyes.

This leads me to believe the color, which is the coloring agent itself, mixed as it is in the plastic in order to produce a singular result, a red pen, is why the color is in the pen.

In scientific terms: the properties of the material in the pen determine the wavelength and efficiency of light absorption, and therefor the color. My question is: what properties in the “color percept”, whether added, removed, or changed, can explain why the pen is red? — NOS4A2

Colour sensations occur when there is neural activity in the visual cortex. These explain dreams, hallucinations, variations in colour perception, and allow for visual cortical prostheses. They also occur in ordinary, everyday experiences, caused by electromagnetism stimulating the eyes. This has all been experimentally verified.

And that is all there is to our ordinary, everyday understanding of colour.

Using the term "colour" in other ways, e.g. as an adjective to describe pens that reflect certain wavelengths of light, or as another term for a colouring agent, does not refute any of the above, and is certainly not the use that is relevant to either the OP's question or the philosophy of colour in general. See for example the SEP summary quoted here.

Yet we both see the red in the pen. — Banno

There is no red "in" the pen. The pen just has a surface layer of atoms that reflects light with a wavelength of ~700nm. When light stimulates the eyes it causes the neurological activity responsible for colour percepts, and we name the colour percept ordinarily caused by 700nm light "red".

You are being deceived by the brain's ability to make it seem as if the qualities of visual experience extend beyond itself, like being convinced that your phantom limb is real. Physics and the neuroscience of perception have proven this naive realism false.

How do you know your memory is sufficient? Because you remember? Somewhat circular, don't you think? — Banno

Perhaps, but sufficient nonetheless. I know that today’s headache is much like yesterday’s headache and that I hid my toys under the floorboards of my childhood home. I don’t need verification from other people to trust that I remember correctly.

Sure we talk about pain, and so far as we do it is not private. — Banno

Then insofar as we talk about our colour percepts they are not private; but they are nonetheless percepts and not mind-independent properties of pens.

If we agree that this pen is red, and the others are not, then we agree to something about this pen, and not to something that is only in your mind. — Banno

And if we agree that stubbing one’s toe is painful and that hugs are not then we agree to something about stubbing one’s toe; but pain is still a mental percept.

So once again, the fact that pens are red simply does not entail that colours are mind-independent.

It's that thinking about it in terms of things being mind-dependent or mind-independent is muddled, and can best be replaces by thinking about the actions of embodied people in a shared world. — Banno

Some things are neurological phenomena, some things aren’t. Pain is a neurological phenomenon, smells and tastes are neurological phenomena, and colours are neurological phenomena. Pens may have atoms that reflect light, but this physical phenomenon simply isn’t what we think or talk about when we think and talk about colours. We may mistakenly believe that colours are properties of pens, and talk about them as if they are, but we would simply be wrong. The science is clear on this, and no deferment to Wittgenstein can show otherwise.

Quite a bit. If your "mental percepts" are individual, in your mind only and unsharable, then they are tantamount to the private sensation "S" used by Wittgenstein. You might now be calling "red" the percept you yesterday called "green"; you have no way of checking except your own memory. — Banno

My memory is sufficient. I have every reason to believe that today's headache is much like last yesterday’s headache. I don't need some other person to talk to at all.

The way we talk about colours and pains are different. They involve, in Wittgenstein's terms, different grammars. — Banno

English grammar does not determine what's true and what's false. That we talk about colours as if they are mind-independent does not entail that they are mind-independent. Physics and the neuroscience of perception have proven that our presuppositions are wrong.

And pain works somewhat differently to colour. There is no equivalent to the box, no something that is available for us both to examine. — Banno

And yet we can, and do, talk about pain, which you seem to admit is a private sensation. If you admit of private sensations that our words can refer to then your private language argument fails.

Not quite. The argument is more that you and I can both choose the red pen from a container of various other colours, and hence that we agree as to which pen is red, and that hence being red is different to being black or blue - and that this is a difference in the pens, not just or only in your mind. We agree as to which pen is red and so being red involves pens as well as sensations. — Banno

And you're back to using "red" as an adjective. That pens are red and that pens are mind-independent is not that colours are mind-independent. You continue to repeat the same non sequitur.

All this means is that scientists use that term to talk about seeing colours. — Banno

Which can occur without some "appropriate" distal object reflecting light into our eyes. Seeing colours occurs when the visual cortex is active, and these colours seen are not mind-independent properties of pens. These percepts constitute our ordinary, everyday conception of colours (and even if we're naive realists who mistake them for being something else).

As described above, this is not in keeping with the present scientific view. — frank

The present scientific view is that colour percepts exist, and do so when there is neurological activity in the visual cortex. This is what explains dreams, hallucinations, and variations in colour perception, and allows for visual cortical prostheses.

And it is these percepts, not a surface layer of atoms reflecting various wavelengths of light, that constitute our ordinary, everyday understanding of colours (even if we naively and mistakingly think these percepts to be or resemble some mind-independent property).

See the SEP summary I posted .

My point is that you need both internal and external data to distinguish between colors. — frank

Which is wrong, because I don't (except insofar as an external stimulus is causally responsible for the sensation).

All I need is visually distinguishable percepts (whatever their cause). Animals can distinguish between the poisonous red frog and the non-poisonous brown frog without having to converse with one another.

You really need to move past this language-first approach to biology.

If that was true you would have easily been able to pick them out in the apple picture. You need an external crutch to distinguish between them. — frank

I don’t even understand what you’re asking. What do you mean by “pick out”? Are you asking me to name each hue without using words? Obviously I can’t do that because your request is nonsensical.

But I can visually see that the apple has a gradient of hues ranging from lighter to darker to lighter, and isn’t just a single solid hue like the second rectangle I posted. And I can see this despite not having individual names for each hue, proving my point and refuting yours.

You see the shades of red, but you can't distinguish between them without an external crutch. — frank

I can distinguish between them. That’s how I can see 5 hues in that first image. It’s not just a single hue like the second image.

This obsession that you and others have with Wittgenstein and language is a hopeless confusion. Colour experiences, like other experiences, concerns sensory percepts, and often the sense organs and stimuli that they react to. It doesn’t concern speech or writing.

Both of those would demonstrate color externalism. — frank

No it wouldn’t. We’d just have words that refer to individual hue percepts.

Without looking at your sample, identity each of those shades in this picture... without any words. — frank

I don’t need words to see that there are lighter and darker shades of red. I don’t need language to see colour at all because seeing colours does not depend on language.

I don’t have individual names for shades. What is the relevance of your question?

If you use the same word for both, that might diminish your awareness of a distinction, right? — frank

I can distinguish shades of red.



I can see that there are 5 different colours (or hues if you prefer), not just a single red rectangle like below:

Our eyes are detecting and responding to the same wavelengths of light. We see different colours because our brains react differently to the signals sent from the eyes, producing different colour percepts, and seeing colours is the occurrence of these colour percepts.

The science is clear on this. And thankfully so, as we are working on visual cortical prostheses that use direct electrical stimulation of the visual cortex, bypassing the eyes, to hopefully allow the blind to see (and in colour).

What does Wittgenstein's private language argument have to do with anything we're discussing here? We have words like "pain" and "sensation" that refer to things like pain and sensation. So either a) pain and sensation are not private things or b) our words can refer to private things. But also, of course, the phrase "private things" refers to private things.

Either way, it makes no sense to try to use Wittgenstein to prove that colours are not a type of sensation, comparable in kind to pain.

But more than that, as it stands your reasoning seems to amount to nothing more than "pens are red, pens are mind-independent, therefore red is mind-independent." This argument is a non sequitur, exactly like the analagous argument "stubbing one's toe is painful, stubbing one's toe is mind-independent, therefore pain is mind-independent."

The SEP article on fictionalism seems relevant here:

Here is a kind of puzzle or paradox that several philosophers have stressed. On the one hand, existence questions seem hard. The philosophical question of whether there are abstract entities does not seem to admit of an easy or trivial answer. At the same time, there seem to be trivial arguments settling questions like this in the affirmative. Consider for instance the arguments, “2+2=4. So there is a number which, when added to 2, yields 4. This something is a number. So there are numbers”, and “Fido is a dog. So Fido has the property of being a dog. So there are properties.” How should one resolve this paradox? One response is: adopt fictionalism. The idea would be that in the philosophy room we do not speak fictionally, but ordinarily we do. So in the philosophy room, the question of the existence of abstract entities is hard; outside it, the question is easy. When, ordinarily, a speaker utters a sentence that semantically expresses a proposition that entails that there are numbers, what she says is accurate so long as according to the relevant fiction, there are numbers. But when she utters the same sentence in the philosophy room, she speaks literally and then what she asserts is something highly non-trivial.

Some seem to insist on the fiction, denying the sense in asking deeper questions.

We all use them to pick out white and gold and blue and black things. We just differ on which things. — creativesoul

See what I said to Banno about the distinction between the adjective "red" and the noun "red":

The adjectives "red" and "painful" describe things like pens and stubbing one's toe.

The nouns "red" and "pain" refer to the mental percepts that pens and stubbing one's toe cause to occur. — Michael

Colours, as ordinarily understood in everyday life, are how things look, not how things reflect light. How things reflect light determines how things look, and so determines the colour seen, but reflecting light is distinct from colour.

When I think about the colour red I am not thinking about light reflectances; I am thinking about the visual percept.

Do all of the eyes that are perceiving the very same scenery at the very same time from nearly the same vantage point perceive the same light? Yup. — creativesoul

We see the same light but not the same colour. Therefore the light is not the colour. The light is the cause of the colour (much like the chemicals in the food are the cause of the taste), nothing more.

Carrying on from this, here are two different claims:

1. An object is red if it looks red
2. An object looks red because it is red

With the first, a sentence such as "the pen is red" just means "the pen looks red", and the word "red" in "the pen looks red" refers to the mental percept.

With the second, one must explain the "because it is red" part.

One offered explanation is dispositionalism, which gives us something like "the pen looks red because it is disposed to look red". But again, the "red" in "disposed to look red" refers to the mental percept.

Another offered explanation references wavelengths of light, which gives us something like "the pen looks red because it reflects 700nm light". Unlike previous examples, this is no longer a semantic claim, but an empirical claim, and in most cases it is true, but given variations in colour perception and so-called colour "illusions", a pen can look orange even though it reflects 700nm light, and so to reintroduce the terminology given in (2), a sentence such as "the pen looks orange even though it is red" is both meaningful and can be true. In this case, the word "orange" is referring to the mental percept and the word "red" is referring to the reflectance.

But also in this case, the use of the terms “because” and “even though” are questionable, as there’s nothing a priori wrong with these claims:

3. The pen looks red even though it reflects 700nm light
4. The pen looks orange because it reflects 700nm light

And so nothing a priori wrong with these claims:

5. The pen looks red even though it is red
6. The pen looks orange because it is red

Why focus on color specifically then? — Hanover

Because that's what this discussion is about. We're accepting realism in the general sense; atoms exist, reflecting wavelengths of light, and trees are a particular collection of atoms. We then want to know if colours are, as the naive realist believes, mind-independent properties of trees, or if they are mental percepts like pain.

As summarised by the SEP article quoted here, and as mentioned in the several scientific studies I've referenced, the physics and neuroscience is clear that colours are mental percepts, and I am going to trust what physics and neuroscience tell us about the world and perception (and certainly over a philosopher of language like Wittgenstein).

I'm just not buying into Lockean primary and secondary qualities where some qualities are deemed mind created and others inherent in the object. — Hanover

But you just did with pain? You accept that pain is a mental percept. Presumably you accept that trees are not a mental percept?

Or are you actually an antirealist/idealist, rejecting mind-independence entirely? Because that seems like a matter for a different discussion.

Not the same article, but this one is free to read:

Colour variation is the fact that what colour physical objects look to have depends on viewing conditions and a perceiver’s visual system. Both Colour Relationalists and Colour Eliminativists regard their analyses of colour variation as central to the justification for their respective views. Yet the analyses are decidedly different. Colour Relationalists assert that most instances of colour variation are veridical and infer from this that colours are relational properties of objects that are partly determined by perceivers. By contrast, Colour Eliminativists assert that colour variation is too unsystematic to ground the claim that many or most instances of colour variation are veridical. From this they infer that objects don’t have colours. I argue that the Eliminativist analysis is superior. On my view, the Relationalist account of veridical colour experience reduces to the assertion that objects have colour simply because they cause perceivers to have colour experiences of them. In this context, I argue, the resulting conception of veridicality is vacuous. More directly, the foundational idea of Eliminativism is the opposite claim: the fact that objects cause perceivers to have colour experiences of them is on its own not sufficient to justify or ground the claim that objects have colour. The Relationalist, I argue, has failed to justify anything stronger than this. In this debate we should thus side with the Eliminativist: objects do not possess colour; they merely cause us to undergo colour experiences.

And this quote may be of interest to @Banno and @Hanover:

It is fair to say that Eliminativists value adequate explanations of phenomena like colour variation more than, for example, offering a straightforward account of Common Sense Colour. In this regard, theirs is a “perception first” approach to colour (as opposed, e.g., to a “language first” or “no priority” approach). They need not (and generally do not) doubt that there are blueberries, that blueberries often cause perceivers to undergo colour experiences, or that our concept BLUE is often meaningfully applied, and to good effect, to blueberries. What they doubt is that blueberries are blue in a basic sense, believing instead that blueberries merely have the power to cause perceivers to undergo colour experiences. Insofar as we can explain what basic colours are, blueberries only have this causal role. No feature of blueberries is part of the basic nature of colour or of what constitutes basic colour.

Sure. The relevance of that distinction here, however, escapes me. — Banno

The adjectives "red" and "painful" describe things like pens and stubbing one's toe.

The nouns "red" and "pain" refer to the mental percepts that pens and stubbing one's toe cause to occur.

Pain and colour are different. I can hand you the pen, but not the pain. — Banno

You cannot hand me the pain or the colour.

Why should there be a singular thing to which the noun "colour" refers, and which must therefore be either in your head or in your hand? — Banno

You used the noun "colours" to say that colours are more than just mental percepts. This is equivocation, like using the noun “trees” to say that trees are both woody perennial plants and branching diagrams.

This is different to simply saying that the nouns “colours” and “trees” can refer to more than one thing, which I have repeatedly accepted.

But colours as ordinarily understood in everyday life are mental percepts, not reflectances, and trees as ordinarily understood in everyday life are plants, not diagrams.

It is obvious in context that the OP is not asking if atoms reflecting light is mind-independent.

Do you believe that pain is a mental percept or a mind-independent property of distal objects? If the former, does this lead to something like idealism? If not, why do you think that the claim that colours are also a mental percept leads to something like idealism?

I just don't understand your reasoning at all.

Some things, like pain, are in the head. Other things, like trees, are not. The science shows that colours, as ordinarily understood, are in the former group, not the latter. None of this entails abandoning realism entirely.

Yet it is more plausible to believe that it is the addition of a substance that causes the variation. There is no good reason to believe that the variation occurs without the added substance. — jkop

And adding cold water to boiling water means I no longer feel pain when I put my hand in the water. That doesn’t entail that pain is a mind-independent property of (boiling) water.

All you are explaining is what I already accept; that mind-independent properties are causally responsible for percepts, and so changing those properties will change which percepts are caused to occur - because the brain reacts differently to different stimuli.

For example, you claim that the colour variation in the dress is caused by the brain. — jkop

Because it is. I’ve already referenced actual scientific studies on the matter.

My colleague and I are looking at the same computer screen and the same light is striking our eyes. Yet we see different colours because our brains process the stimulus differently. This is a proven empirical fact.

A summary of what? The article contains many different sections and summaries, and you picked one that partly (debatable) suits one or two of your single-minded assertions. :roll: — jkop

I agree with the part I quoted, which is why I quoted it as being what I believe is correct. I don’t agree with the competing theories that I didn’t quote. I trust physics and neuroscience over armchair philosophy.

The “common-sense” naive view truly posits that colors are mind independent properties of objects because when I change the color of my room’s wall and get another bucket of paint with a different color, not a different mental percept. — Richard B

Some paint reflects 700nm light, which causes us to see red. Some paint reflects 450nm light, which causes us to see blue. Painting your room changes which wavelengths of light are reflected.

But colours as ordinarily understood are colours-as-we-experience-them, not micro-structural properties that reflect various wavelengths of light, and these colours-as-we-experience-them are mental percepts that various wavelengths of light cause to occur.

I intended this post above to be my final post, so I'll leave it there.

It is strange to ask if mental percept are mind-independent — Richard B

You're misunderstanding.

The Morning Star is a planet, but it is perfectly appropriate to ask if the Morning Star is a planet or a star (e.g. if one is unsure).

Our ordinary conception of colours is that of colours-as-we-experience-them, which contrasts with such things as colours-as-dispositions-to-reflect-light. The "common-sense" naive view falsely posits that colours-as-we-experience-them are mind-independent properties of trees and pens and chairs, but the science shows us that they are not; that they are mental percepts related to neural activity in the visual cortex.

Problem with this is you are no longer talking about mind dependent concepts but mind independent (brain neurons etc I would think you would call mind independent). — Richard B

If the mind is brain states then to say that something is mind-independent is to say that it is independent of brain states. Brain states are not independent of brain states.

So if colours are mental phenomena and if mental phenomena are brain states then colours are brain states. Brain states are not properties of trees and pens and chairs, and so colours are not properties of trees and pens and chairs.

I'll finish my time here by quoting the SEP article again. I believe this summary is correct:

One of the major problems with color has to do with fitting what we seem to know about colors into what science (not only physics but the science of color vision) tells us about physical bodies and their qualities. It is this problem that historically has led the major physicists who have thought about color, to hold the view that physical objects do not actually have the colors we ordinarily and naturally take objects to possess. Oceans and skies are not blue in the way that we naively think, nor are apples red (nor green). Colors of that kind, it is believed, have no place in the physical account of the world that has developed from the sixteenth century to this century.

Not only does the scientific mainstream tradition conflict with the common-sense understanding of color in this way, but as well, the scientific tradition contains a very counter-intuitive conception of color. There is, to illustrate, the celebrated remark by David Hume:

"Sounds, colors, heat and cold, according to modern philosophy are not qualities in objects, but perceptions in the mind. (Hume 1738: Bk III, part I, Sect. 1, [1911: 177]; Bk I, IV, IV, [1911: 216])"

Physicists who have subscribed to this doctrine include the luminaries: Galileo, Boyle, Descartes, Newton, Thomas Young, Maxwell and Hermann von Helmholtz. Maxwell, for example, wrote:

"It seems almost a truism to say that color is a sensation; and yet Young, by honestly recognizing this elementary truth, established the first consistent theory of color. (Maxwell 1871: 13 [1970: 75])"

This combination of eliminativism—the view that physical objects do not have colors, at least in a crucial sense—and subjectivism—the view that color is a subjective quality—is not merely of historical interest. It is held by many contemporary experts and authorities on color, e.g., Zeki 1983, Land 1983, and Kuehni 1997. Palmer, a leading psychologist and cognitive scientist, writes:

"People universally believe that objects look colored because they are colored, just as we experience them. The sky looks blue because it is blue, grass looks green because it is green, and blood looks red because it is red. As surprising as it may seem, these beliefs are fundamentally mistaken. Neither objects nor lights are actually “colored” in anything like the way we experience them. Rather, color is a psychological property of our visual experiences when we look at objects and lights, not a physical property of those objects or lights. The colors we see are based on physical properties of objects and lights that cause us to see them as colored, to be sure, but these physical properties are different in important ways from the colors we perceive. (Palmer 1999: 95)"

This quote, however, needs unpacking. Palmer is obviously challenging our ordinary common-sense beliefs about colors. Specifically, he is denying that objects and lights have colors in the sense of colors-as-we-experience-them (or colors as we see them), As far as this goes, it is compatible with objects and lights having colors in some other sense, e.g., colors, as defined for scientific purposes. Secondly, he is saying that color (i.e., color-as-we-experience it) is a psychological property, which in turn, might be interpreted in different ways.

I trust physicists and neuroscientists over Wittgenstein.

falsely claim that the reason we see different colours is to be found in the brain — jkop

It is.

What's your view of that? — frank

That it's wrong. The word "percepts" refers to percepts, the word "pain" refers to a subset of percepts, and the word "colour" refers to a different subset of percepts.

This Wittgensteinian approach that wants to explain all language in terms of some public behaviour just doesn't work, so move on from it. Some words refer to other things.

Well, as a nominalist I don't buy into universals. But the existence or non-existence of universals seems like matter for a separate discussion.