Color-coding has the central problem that colors stand for more than just racial categories. Black is still within spitting distance of acceptability because it's fairly commonly used, but Yellow? Red? Not helping. All of those three colors stand rather prominently for negative characteristics in common use that have nothing to do with race.
I don't get the sense that Americans are particularly concerned about the many seams and cracks in the prominent racial categories we use. The idea of the country as a melting pot really does shape our racial perceptions. We have it in our heads that the natural and correct progression of immigrants is assimilation. The idea of subdividing our existing racial categories for the sake of accuracy is a step backward in our minds. Each category represents a possibility for friction, so the fewer categories the better. Italians, English, and Germans have certainly gotten along better in the US since they all started being plain old Caucasians, after all.