Within the context of continental Enlightenment epistemological philosophy, imagination is a faculty in use both a priori and a posteriori. In the latter, imagination synthesizes appearances to phenomena, and passes to understanding representations of real objects as a mental images, which become our experiences. In the former, imagination spontaneously creates its own intuitions, and synthesizes these to form a representation which it then passes to understanding, not as a mental image but as schema, to which belongs the content of our pure conceptions, re: geometric figures, numbers, colors.
(CPR, B178......loosely....it’s pretty forbidding down here in the weeds)

Given an experience of blue, it is not a problem to imagine a shade not belonging to experience, re: Hume’s missing shade in E.C.H.U., Sec 2. Schema exist only in thought, which makes explicit that if there is experience of a blue, which is technically an intuition of an object with the property of blue inhering in it, as understanding thinks as belonging to it, we can easily imagine a blue of more or less degree, even in that very same object.

Depending on which doctrine one accepts, color either belongs to an object which we then perceive, or color resides in us merely as a pure conception, like extension, size, causality, which we apply to objects as part of our cognitive process.

Anyway.....I know. It doesn’t help when the answer to a question just raises more of them.