A good question. The disputable value is 'maximum possible perceptual hue difference'. In color science, difference is usually understood in terms of 'minimum

A good question. The disputable value is 'maximum possible perceptual hue difference'. In color science, difference is usually understood in terms of 'minimum possible'. This is how the god-like Albert Munsell first mapped the perceptual color domain. Maximum possible has almost no presence in color science literature. I assume that this is because of the asymmetry of perceptual maximum/perceptual minimum. Without solid empirical data, I can only support the validity of RYB pairs by pointing out that Leonardo (and everyone else who wrote about color prior to Newton), supported this pairing without knowing the relative position of these pairs on any color wheel. A weak argument, perhaps. I know it is possible to design an experiment to prove/disprove the validity of RYB complementaries. It was on our to-do list before our computational aesthetics project ran out of money. However, I shall continue to assume its truth. My reason is simple: from a life time as a painter I know that RYB complementary pairs are true. An even weaker argument.