Comment by labawi

Yes, color is a human concept for certain stimuli, no recording or reproduction is 100% accurate and even the model is not perfect (different people have shifted color sensitivities, some are even tetrachromats and we are ignoring rods vs. cones altogether).

However, the stimuli is predominantly light, which can be quantified and certain operations with light are well studied. Not all, but most noticeable ones are and that is what is used when doing photorealistic rendering etc.

If you do a 2x downscale, you need to average 4 pixels together. Linear RGB should (by theory and definition) give you a (theoretically) physically accurate result, as if you did a physical 2x downscale by putting it further away in a uniformly lit room or whatever. You can't reproduce that precisely, but neither can you reproduce 1l + 1l of milk = 2l of milk.

This is in stark contrast to downsizing, blur or whatever "performed" in sRGB, where images get curiously darker and slightly off-color (or is it lighter?).

I'm not sure what is perceptual average of colors, but if you mix pixels in 1:1 ratio and apply blur / look from far away, there is only one correct result, and linear RGB average (is one way that) gives you the single correct result. (ignoring screen color reproduction deviations from what they are supposed to be)