How do we know that your ‘red' looks the same as my ‘red'? For all we know, your ‘red' looks like my ‘blue'. In fact, for all we know your ‘red' looks nothing like any of my colors at all! If colors are just internal labels, then as long as everything gets labeled, why should your brain and my brain use the same labels?
Richard Dawkins recently wrote a nice little piece on color, and along the way he asked these questions.
He also noted that not only can color labels differ in your and my brain, but perhaps the same color labels could be used in non-visual modalities of other animals. Bats, he notes, use audition for their spatial sense, and perhaps furry moths are heard as red, and leathery locusts as blue. Similarly, rhinoceroses may use olfaction for their spatial sense, and could perceive water as orange and rival male markings as gray.
However, I would suggest that most discussions of rearrangements of color qualia severely underestimate how much structure comes along with our color perceptions. Once one more fully appreciates the degree to which color qualia are linked to one another and to non-color qualia, it becomes much less plausible to single color qualia out as especially permutable.
Few of us, for example, would find it plausible to imagine that others might perceive music differently, e.g,. with pitch and loudness swapped, so that melody to them sounds like loudness modulations to me, and vice versa. Few of us would find it plausible to imagine that some other brain might perceive ‘up' (in one's visual field) and ‘down' as reversed. And it is not quite so compelling to imagine that one might perceive the depth of something as the timbre of an instrument, and vice versa. And so on.
Unlike color qualia, most alternative possible qualia rearrangements do not seem plausible. Why is that? Why is color the butt of nearly all the "inverted-spectra" arguments?
The difference is that these other qualia seem to be more than just mere labels that can be permuted willy nilly. Instead, these other qualia are deeply interconnected with hosts of other aspects of our perceptions. They are part of a complex structured network of qualia, and permuting just one small part of the network destroys the original shape and structure of the network - and when the network's shape and structure is radically changed, the original meanings of the perceptions (and the qualia) within it are obliterated.
The reason other qualia seem to be more than mere labels is that most of them have clear meanings and functions. We know what they're for, and how they plug in to the rest of our network of qualia. For color, on the other hand, we have historically been largely blind to what colors are for, and how they functionally integrate with the rest of our perception. In the absence of knowing how to plug colors in to the rest of our qualia, they do seem much more rearrangeable.
But we're beginning to know more about what colors are for, and as we learn more, color qualia are becoming more and more like other qualia in their non-permutability. Let's see why.
First, even before describing some of the new insights on color vision, I note that most conversations about color qualia don't seem to account for what has long been known about colors. Colors are not a set of distinct crayons with no connections to one another. Instead, colors are part of a three dimensional space of colors, a space having certain well-known features. The space is spanned by a red-green axis, a yellow-blue axis, and a black-white axis. These three axes have opponent colors at opposite ends, and these extreme ends of the axes are pure or primary (i.e., not being built via a combination of other colors). All the colors we know of are a perceptual combination of these three axes. For example, burnt orange is built from roughly equal parts yellow and red, and is on the darker side of the black-white dimension.
To perceive colors like I do requires, at a minimum, having the same color space as I do. To perceive ‘red' without having (its opposite) ‘green' also as part of one's color space is impossible, just as perceiving ‘light' would be impossible without also having ‘dark'. And to perceive orange without having both red-green and yellow-blue axes is impossible, because orange is a perceptual mix of red and yellow.
And that's just the bare beginnings of the structure of colors. Colors are not only intricately connected to one another in a space, but are linked to many other aspects of our mental life, including other sensory modalities (e.g., a "red sounding trumpet") and emotions.
In fact, in my research I have provided evidence that our primate variety color vision evolved for seeing the color changes occurring on our faces and other naked spots. Our primate color vision is peculiar in its cone sensitivities (with the M and L cones having sensitivities that are uncomfortably close), but these peculiar cone sensitivities are just right for sensing the peculiar spectral modulations hemoglobin in the skin undergoes as the blood varies in oxygenation. Also, the naked-faced and naked-rumped primates are the ones with color vision; those primates without color vision have your typical mammalian furry face.
In essence, I have argued elsewhere that our color-vision eyes are oximeters like those found in hospital rooms, giving us the power to read off the emotions, moods and health of those around us.
On this new view of the origins of color vision, color is far from an arbitrary permutable labeling system. Our three-dimensional color space is steeped with links to emotions, moods, and physiological states, as well as potentially to behaviors. For example, purple regions within color space are not merely a perceptual mix of blue and red, but are also steeped in physiological, emotional and behavioral implications - in this case perhaps of a livid male ready to punch you.
Furthermore, these associations are not arbitrary or learned. Rather, these links from color to our broader mental life are part of the very meanings of color - they are what color vision evolved for.
The entirety of these links is, I submit, what determines the qualitative feel of the colors we see. If you and I largely share the same "perceptual network," then we'll have the same qualia. And if some other animal perceives some three-dimensional color space that differs radically in how it links to the other aspects of its mental life, then it won't be like our color space. ...its perceptions will be an orange of a different color.