if you want to draw as little attention as possible to yourself while taking pictures or want that harmonious look with black dslr body, i'd suggest you'd get black.
with that said, i personally own silver 77mm and 43mm; they simply look gorgeous. also going along with pentax's "be different" motto, silver limited truly are unique; you see black lenses everywhere but not the silver ones!
silver on black body does not bother me at all, but i believe this is a personal thing and some may be bothered by it.
here is an image i found in one of korean pentax forums, which shows one lucky guy with beautiful set of pentax silver lenses. makes me so jealous