I love stamping on polymer clay with Staz-On inks, because they "staz-on"!
Here is a link to a switch cover that I did - https://www.flickr.com/photos/thepleasantpheasant/16505910171/in/dateposted-public/. There are a few other switch covers near that one on my Flickr that use the same techniques. You can use a lot of things to stamp on clay - for white on dark, I would try some white Pinata alcohol ink - although practice would be the thing before trying it on a final piece, since it would be messy. Also, the color of the ink would probably stay on the stamp pad, although it wouldn't probably come off in future stamping sessions. To be sure of that, spray the stamp with rubbing alcohol (or just wet a rag with alcohol) and make sure no color is coming off on the rag.
You can also shred chalk on to clay and rub it in (from experience -stick with one light color, then add a few streaks of some other chalk color. Too many will turn to mud color.) Once you bake it, it's permanent.
In fact, that is chalk under the stamps on the light switch cover link. :)
I hope this helps! Most important - have fun!!