Thanks, that clears up one thing I was puzzling about: if I were asked to give a quantitative value to how different two colors seem to be, my first problem wou

Thanks, that clears up one thing I was puzzling about: if I were asked to give a quantitative value to how different two colors seem to be, my first problem would be to figure out what a number - any number - would mean in this context. While 'barely perceivable difference' seems like a reasonable unit of measure, I don't think I could judge multiples of that unit with any degree of accuracy or consistency, or, indeed, in any meaningful way. On thinking about this issue, it seems to me that I think of large differences categorically (red vs. green, for example) while thinking of small differences as being on a continuum, even when the colors in question lie in the borderlands of the above categories.