If there is such a simple argument for physicalism, how come everybody hasn't always been a physicalist? That's a good question, and there is a good answer. The 'causal completeness of physics' wasn't widely accepted until recently.

I don't think that we can figure out what is going on in conscious colour perception just by phenomenological introspection. We need to know about brain mechanisms as well. We need to figure out what information is present in the mechanisms that constitute conscious colour perception.

I'm not so sure that I am a reductionist in the strict type-identity sense. The issues here are messy. But I certainly a reductionist in the more general sense which is opposed to eliminativism and dualism.

Of course, there remains the question of why we should find mind-brain identities so persistently counter-intuitive, if they are true. But this is a simple psychological question, and there are a number of plausible explanations. Indeed this is a topic that is quite extensively discussed outside philosophy, by developmental psychologists and theorists of religion among others, under the heading of 'intuitive dualism'. It is rather shocking that so few of the many philosophers working on 'the explanatory gap' are familiar with this empirical literature.

I don't think that we are capable of anything like this many possible colour responses. Instead I argue that the perception of colour differences between two surfaces viewed side-by-side is a gestalt phenomenon.

A century ago mainstream science was still quite happy to countenance vital and mental powers which had a 'downwards' causal influence on the physical realm in a straightforwardly interactionist way. It was only in the middle of the last century that science finally concluded that there are no such non-physical forces. At which point a whole pile of smart philosophers (Feigl, Smart, Putnam, Davidson, Lewis) quickly pointed out that mental, biological and social phenomena must themselves be physical, in order to produce the physical effects that they do.

There is a brain mechanism that works to identify colour differences directly, without first identifying the absolute colour of each surface. So on my view there is no reason to suppose anything like ten million colour responses to surface viewed singly.