I believe there are enamels or ground glass that can be painted on glass and
There are colours for work with enamel or pottery, which are
simply metal oxides. Traditionally, they are worked with
sandalwood oil to paint miniatures onto enamelled surfaces. The
oil has to dry properly, which will take some time, before
firing. There are even gold and platinum "colours" - the real
metals - for painting and firing on chinaware, they should work
with enamel, too.