The term comes from an old blog post [0] about different kinds of effect systems. Every function has a color, and every colored function can only call functions that are compatible with it, usually of the same color. The net result is that you end up either duplicating a lot of your common code so you have compatible interfaces for all the different colors (let's call that "separate but equal" if we're feeling spicy), or you end up shoving round pegs into the square holes of your dominant function color.

[0] https://journal.stuffwithstuff.com/2015/02/01/what-color-is-...