OK, here's where my stained glass background finally comes in handy. The Pebeo brand paints that are sold at Michael's are pretty good. Make sure you use the Vitrea 160 series. These are specially made for glass. You can get them as small bottles of paint or as paint markers. They do need to be baked in your oven (not a kiln). When baked in the oven, the paint is very difficult to get off. I tried a test piece once and tried to scrape off the paint using a razor blade scraper. I barely made a dent.