This is an update on the Biting the Bullet for Physicalism entry I wrote in September. I was recently moving entries from another blog to this one. But none of the comments were lost. Adfero Affero said on the original post that,
Qualia, the redness of red, can't be approximations. Whatever the redness of red is, it must be an exactitude because colours are precise differentiators. Red is always distinguished from blue, whatever the redness of red or the blueness of blue are. Though various shades of red might be difficult to distinguish from each other, this can be explained by saying the mixtures of wavelengths of light in the various shades of colour can fool the brain into mistaking one colour for another.
Color does not exist on its own. It has its physical analogues in particular wavelengths in the visible spectrum. So colour (the redness of red) whatever it is must be analogical. To stretch it even further, we might say that the redness of red is a spatio-temporal metaphor. Or even further, that each colour is a fixed story the brain tells itself about a particular wavelength of light. So when you can't tell one colour from another, the story being told is too similar, hence mistakes are made in telling one from the other. Though of course the eye and the brain are processing wavelengths of light which appear to be colours, which are distinguishable one from another, though we don't know what colour is except that it represents wavelengths of light.
A flock of birds is not an approximation, though flock is a category word which does not specify how many birds or what they are up to, merely that it is all birds. The flock you see at any one moment contains a countable number of birds, doing a theoretically measurable number of things, for exact periods of time. Approximation only comes in because we can't be bothered to find a way of understanding everything that is going on in a flock at a given moment.
I'm a little confused as to what the point about the color spectrum is getting at. Or whether it's a question. But epiphenomenalists and dualists do invoke the inverted qualia (and the inverted color spectrum) to argue that qualia are non-physical. They say we can imagine a scenario in which there are differences in experience but which have no physical or causal properties. So qualia change, but physics (and everything reducible to it) does not.
But if you hold to the view that physics is complete, then there is a physical answer. If there is a spectrum inversion, then there is a physical difference. If qualia are physical, then there cannot be a situation in which quale experiences in two identical observers do not have identical experiences.
There is a knowledge argument made in response to this, which says that you would never know if they were not identical--since they would say the same things about their experiences. But that doesn't matter much because what concerns us is possibility, not behavior. It would not be possible for those experiences to be different. It would be like saying a "married bachelor" is possible.