I literally just painted it at one of those ceramic art workshops. The mug is standard clay, fired in a kiln, so it is usable and machine washable. Next when I visit the workshop next Thursday to pick up my latest piece I'll check what paints I've been using, and let you know.
The paint they (and incidentally I) used is called Duncan Concepts, described as an Underglaze for Bisque. They have a variety of colors marked separately by the letters CR followed by a 3-digit number. For future projects, I will be listing which numbers I used and how, so that others can see how my art turned out. Bisque is the type of ceramic I am working on, which is an unglazed white ceramic ware. After painting the piece, the ceramic art shop glazes the piece and fires it in a kiln, setting the colors in and protecting the pottery afterwards.