Suppose that colors have a fourth dimension to which we, humans, are not sensitive. As a matter of fact, I gather that this is true, and that certain birds are now thought to be sensitive to just such an additional dimension. I don't see why this would challenge the existence of the colors we now see. The colors are there, it's just that we're not sensitive to all their aspects. But this answer assumes a sort of "color objectivism"--that is, that colors are properties out there in the world (certain reflectance patterns perhaps) that obtain independently of our seeing them. Suppose instead that we think of colors more subjectively, as existing in our minds--perhaps as qualitative features of our color-experiences. (I don't think this is the correct way to think of colors, but others do, and certainly some of our color-talk seems to embody this view.) If we think of colors this way (call them "subjective-colors"), then the subjective-colors of the birds--what it's like to have their color-experiences-...

This is a natural and important question to wonder about. It is also an old and distinguished one, dating back at least to John Locke. In its modern incarnation it's often called the problem of "spectrum inversion" or "qualia inversion". Two people might make all the same color discriminations, and use color language in all the same ways, so that outwardly (from a third-person perspective) we would have no reason to say that colors don't "look the same way" to them. But how can we be sure? Isn't it possible that a red object looks to me exactly the same way a green object looks to my functional twin, and vice versa? There is currently a raging disagreement about this, and it leads directly into the fascinating and vexing mind-body problem. Much of the debate turns on what we should mean by "the way a color looks" or "the qualitative character" color experience. Some hold that the way things look or seem first-personally does not go beyond the way that a person reacts to, processes, and acts upon her...