Erdal's own novel is called The Missing Shade of Blue, and its title reminds me of this famous passage from Hume:
I believe it will readily be allowed, that the several distinct ideas of colour, which enter by the eye, or those of sound, which are conveyed by the ear, are really different from each other; though, at the same time, resembling. Now if this be true of different colours, it must be no less so of the different shades of the same colour; and each shade produces a distinct idea, independent of the rest. For if this should be denied, it is possible, by the continual gradation of shades, to run a colour insensibly into what is most remote from it; and if you will not allow any of the means to be different, you cannot, without absurdity, deny the extremes to be the same. Suppose, therefore, a person to have enjoyed his sight for thirty years, and to have become perfectly acquainted with colours of all kinds, except one particular shade of blue, for instance, which it never has been his fortune to meet with. Let all the different shades of that colour, except that single one, be placed before him, descending gradually from the deepest to the lightest; it is plain, that he will perceive a blank, where that shade is wanting, and will be sensible, that there is a greater distance in that place between the contiguous colours than in any other. Now I ask, whether it be possible for him, from his own imagination, to supply this deficiency, and raise up to himself the idea of that particular shade, though it had never been conveyed to him by his senses? I believe there are few but will be of opinion that he canI think I have to count myself among the few here. We can conjure up ideas in the usual sense of 'idea', but Hume is talking about ideas as things that can "enter by the eye," which can only be something like bits of light. And there's no conjuring those up. Presumably he means the experience of seeing, say, some particular shade of blue, but that doesn't enter by the eye, and it also can't be conjured up. We can dream we see it, or imagine we know what it would be like, but we surely can't actually create the experience by thinking or willpower. If we think we can it's because the grammar of color seems to intimate it to us, showing the hole in our experience where it ought to go. But this via negativa is no substitute for the real thing. Experiences are events, and ideas are public. The idea of private objects as something like a cross between an experience and an idea is a myth. But not one I expect to go away any time soon, or as a result of reading this post. And I expect I am being much too dogmatic here, hoping for a shortcut to avoid real work.