Feature suggestion

In #9985 a small utility was proposed to brighten or darken a matplotlib color. The utility converted to RGB, to HLS, manipulated the L (lightness) channel, and converted back to RGB.

There was general disagreement about how exactly to do the brightening, (fractional: L*(1+frac), absolute: L + dL, or simply specifying L), the lack of a rgb_to_hls converter was lamented (to mirror the rgb_to_hsv converter, and it was suggested that a color class that simply had methods to change channels in the various color spaces would be the better approach.

Writing such a class was beyond the scope proposed in #9985, hence this issue for discussion. I'd imagine such a color class would require a MEP.