I use white PBT caps on both my home KB and my work KB, and both boards never really look dirty. I'm also pretty obsessive about not typing before washing hands after I've eaten or otherwise get my hands grimy. The rare times I have felt the need to clean the caps, I just use rubbing alcohol and a paper towel and rub them down.
I also have a board with black PBTs that I no longer use, but the keys looked so dirty, all the time, even mere months after cleaning. I liken it to black and white cars...I've owned both of those, too, and it's so much harder to notice dirt on a white car.