Mind and brain are two different categories. If they were one and the same category, we wouldn’t be able to distinguish between the two, which is a contradiction, because we already do.
There is no question that mind is not the brain. Mind is simply not the brain. The question is what is the RELATION between them. And physicists will tell you – and I am no physicists myself – that mind is caused by the brain. I have no reason to doubt this. Sounds pretty sensible to me.
In other words, color (which is in the mind) is caused by neurochemical processes (which are in the brain) and these in turn are caused by the wavelength of the light that comes in contact with our eyes (which is in the physical world outside of the brain.)
You, on the other hand, appear to think that light has color and that this color causes the color in the mind. This is a matter of physics. Because I’m not a physicist, I can’t comment on it. But I can comment on your reasoning insofar you think that the fact that there is a difference between color and its physical correlates means that color must be physical too. It does not.
You ask “what is color?” I am not sure you know what you’re asking. We already know what color is. We know what kind of experience the word “color” refers to. You need to understand what you’re asking. Are you asking “what is the cause of color?” That would be a different question. But we have answers to that question too. What exactly are you asking?
Existence is a property that is given exclusively to the ideas of what might be real. It is not given to every object of experience. Observations of reality, such as seeing a horse running, do not have such a property. Observations are used to determine the value of this property but they do not have the property itself.
An idea that biological unicorns might be real is not real. But an idea that an idea that biological unicorns might be real might be real is real.
When you lie about what you dreamed last night, for example.