This is a whole area of research (as you probably know). I personally only like HSV colourmaps for visualisations of phase, as it has no boundaries at the edges

This is a whole area of research (as you probably know). I personally only like HSV colourmaps for visualisations of phase, as it has no boundaries at the edges and therefore when you wrap over 2π again you don't get a hard discontinuity. Often you're not actually interested in what the "number" is, but how quickly it is cycling – something that it gets across quite well.