I have seen some colored post front sights that are simply a Patridge sight with a 1/10" or 1/8" inch cube of red, or white, or something else attached at the top of the rear face. It goes flush to the top and to the two sides. It doesn't have to be that thick, of course, and might be less subject to accidental dislocation if it were only 1/16" or even less in thickness. This imitates the King Sight Company's red and white post sights of the late 1930s and postwar era.
No reason why you could not do that with something white, or gold, or stainless, or whatever. If you are handy, you might build up a mass of colored epoxy in the proper location, then pare it to fit with a razor blade or jewelers file.