[diiq]

Re: gray. Yes. I didn't say it was a medium gray, I said it was a neutral gray (neither warm nor cool). But you're absolutely right, this model is merely an approximation of perceptual color. You could instead build a complicated manifold representing perceptual color space, and define a criterion for a cloud of points in that manifold that resulted in pleasing colors, but it would be a lot more work than using the sphere, and you'd not gain that much, I don't think.

Re: how do we know? Well, mostly this system is based on the work of a number of thinkers, starting with Goethe and running through Munsell, Itten, Kandinsky, and a bunch of others, who spent a great deal of time putting colored squares next to each other. They discovered physiological effects of color, like simultaneous contrast. They attempted to learn about psychological effects of color. They admitted that, while some color combinations are attractive only to individuals, there were other sets that seem to have universal appeal. One of the things that those universal combinations had in common was that they centered themselves on the color wheel.

We know, more or less, by science. It is a largely empirical question (put some colors together, see if people like it). You can repeat the experiments yourself, if you like --- you asked 'how can we know'? Either take their word, or try it yourself.

And finally, in reply to the my use of the word 'emotional' --- if I were to say that colors whose center of balance is far from the center axis of the sphere are more surprising, and less comfortable, is that acceptable? I don't mean to say that red and green don't evoke different emotions than yellow and purple --- but they are both comfortable, obvious, familiar combinations; while, say, magenta and brown is less comfortable and takes more work to accept.

[hoopadoop]

Less by science, then.

It seems like a particular combination of colours, say a particular red and green, may be a comfortable, obvious, familiar combination right now, in a specific culture - but 100 years ago may have been shocking and outrageous. And, who knows, in 100 hundred years may again be 'less comfortable'.

Goethe, Munsell, Itten, Albers, Kandinsky may have spent a great deal of time putting coloured squares next to each other, but none of them ever conducted anything like an objective scientific study on the 'aesthetic quality' of a palette. Goethe's theory was based heavily on the work of Aristotle. Munsell's views on aesthetics seemed laughably conservative at the time, let alone now. Neither Albers or Kandinsky, ever put foward a 'theory' for evaluating harmonious colours. Albers said that each colour 'goes' equally well with any other colour. Itten & Kandinsky were obsessed by the occult 'meanings' of colours. Certainly they were genius colourists.. but scientists?

[diiq]

Absolutely true. They were all mystics and goofballs. But I'm not justifying the mysticism. I never mentioned 'blond types' and 'black types', I didn't touch on the 'passion' of red, or the 'purity' of white.

I'm talking about the bits that seem to have held up after 100 years. The bits these men found from observation, rather than mysticism. This is, more or less, the method taught to design and art students today. You can see it in paintings from around the world, and going back hundreds of years. Simultaneous contrast exists and is physiological, not cultural. The vibration of complements is build into our visual system, too.

Saying that these people had some bad ideas doesn't make all their ideas wrong. We all have head about Newton's alchemy --- but we praise him for calculus and optics nevertheless. Kepler had his Harmonices Mundi, but we still can call him a scientist; because sometimes he was.

And if someone comes along and makes color theory even more scientific, that would be great! I would be overjoyed, because so much of what's been written about it is absolute bunk. However, at the moment, this is the best answer to the question I can give; and I think it works pretty well when compared to randomness.