Why don't you just get the Ultimate version that already has all the blank keys? I just ordered one from Newegg. I've been wanting a Das since the first version. 20% off plus $60 from the Unicomp I just sold made me pull the trigger.
Oh! you probably meant coloured keycaps like white, red, or other colors. In that case, you can try a dye job. I'm still waiting for my Rit dye order so I can try it.