Sweat from your palms will do much more to alter the colour of your grips, than gun powder will. Regardless of the material the grips are made from, if they do darken a bit from use, the additional patina usually adds to the look, rather than detract from it. I've got a set of grips on a 1911, that were originally kinda' white, after 15+ years, they've darkened somewhat - look more like old ivory. I like the look better now - not quite so stark.