I tried it first with marker and didn't like the way it looked, it was too dark? not sure how to describe it. but i was going to try paint next (i got about 10 backs for $4 on ebay) but i was feeling lazy, so I decided to try my liquid eyeliner
and the eyeliner can be scraped off so i used a pin to smooth the lines. only down side is because it can be scratched, i'm not sure how durable it'll be for casual wear.