Float a couple of drops of nail polish on water and very carefully swirls the colors together. I found that cold water and a warm room seem to help. Swirl the polish with a toothpick. Then dip the part into the water and the polish sticks on the part. This took some practice, and I used black primer with a drybrush of white over top.
My main problem was doing all of this fast enough so that the polish didn't 'freeze' before adhering. Also the polish can turn into a blob and that isn't very useful.