[solasluaith]

I actually think that in overall perceptual difference, using hue and chroma only is in the same ballpark as luminance only. The relationship between colours would also be totally different as they are arranged on a line vs a circle/polygon in terms of perceptual difference. Would be interesting to compare quantitatively.

Of course that doesn’t tell the whole story because in the end luminance and chroma are not perceived the same way as we tend to associate luminance with more with distance and light and hue and chroma more with the surface properties of the object. (As you mentioned we also tend to treat luminance as a continuous variable and colour as categorical even though both are continuous.) So if you want to use luminance because of this difference, that is a valid, but fundamentally different approach, but I don’t think it would necessarily be superior outright and I’ve tried to make the argument of why I prefer mine.