So what you mean is that any color that is a mixture of two wavelength that trigger two different cones is “close” in some sense to the perceived color of eithe

So what you mean is that any color that is a mixture of two wavelength that trigger two different cones is “close” in some sense to the perceived color of either one of those wavelengths? This doesn’t mean color is “circular” per se, right? I suppose color perception with 3 cones has a sort of circularity when we plot the color space in 2d, but I’m not sure that sufficiently explains our experience of color. Anyway, what you’re saying is that mixtures of two things can be perceived as close to one of the two inputs. The argument holds for any number of cones - if we had 10 cones, we could still trigger the two with the furthest apart wavelength response using a mixture of two separate wavelengths, and the result would be “close” to one of the two. But this also holds for any pair - the mixture of imagined cone #3 and imagined cone #6 would also yield a color that is “close” to #3, and also “close” to #6. Has anyone asked a tetrachromat this question? What would it mean if they disagree that red is close to purple? What would it mean if they agree?