[old_tv_nut]

As the text says, this has to do with the perceptual difference between different colors. Important if you are trying to maximize differentiation of colors in a pie chart or signal lights,but totally irrelvant in choice of primary colors, which depends on the much more primitive level of matching the cone cell stimulations. Note that matching implies equality so no estimation of a difference is relevant.

I have also read speculation that no consistent color space is possible because color difference integration may be path dependent.