Pointing that luminance is the best channel to encode spatial frequency is akin to encouraging greyscale.

It’s just that you can do it with a hue and a non zero saturation if it suits the style you are looking for.

Plus sometimes you have more than on me set of data mixed and it’s useful to be able to distinguish both by using multiple colours.

When you want to plot a categorical distribution you need to encode category by colour, but you want them to appear maximally distinctive.