Improve experience of overriding colors

In response to one of my own questions in !23 (merged). No, it doesn't appear possible to have the description of a customization be different from its docstring. It is possible to improve the experience of overriding colors though.

I had to move the pallets up. I could have used custom-add-frequent-value but I think moving the pallets above everything custom makes more sense. As always, feel free to reject this or change anything you don't like.