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Are we not splitting hairs over semantics here? Yes, 'colour' (as we perceive it) does not exist in external reality but the varying wavelengths of light which give rise to our perceptions absolutely do. Any machine/ animal/alien with the requisite sensors can detect these differences. It's only logical to then label/catalogue/differentiate those frequencies internally to rationalise the wall of 'noise' surrounding us. I guess the analogy is AR, we see the world through a filter (the brain) to pull the most relevant information visually to us for day to day life.

Brightness is varying intensities of light/radiation. Much of that may be invisible to us but it is still very real as any astronaut/nuclear physicist will attest. Where we are tuned to 'see' daylight brightness is a product of intensity of those photons. In that sense brightness is an artifact of the biology of the human eye but it is nonetheless an accurate guage of what is happening in our environment.