and sometimes on other metallics. Has to do with the direction that the pearl and metal flake are laid down and how the light reflects off of them and exacerbated by flex agents and other additives. Can be minimized by painting bumpers at the same time as the rest of the car, but any good paint job (including factory) paints them separately. If you see a Maaco or Earl Scheib (are they still around?) paint job for $199, the bumpers usually match better, but so do all the moldings they painted over ;-)
