I just painted some ceramic ornaments this year. That would be the same finish as your tile I would imagine. What I did was to:
1- wash them in vinegar water and dried good.
2-sprayed them with Krylon Matte finish 1311, and them painted with Americana acrylic paint.
3-When finished painting, I sprayed again with the same Kyrlon 1311. They look great! I felt the paint needed something to adhere to, that's why I sprayed them.

Now, I believe I would buy an extra tile of some kind to practice on(if you don't have an extra one. I would try painting one to see how it does, and go from there.

Also, I do believe they have acrylic paint for glass. You could probably use that paint too. In my area, acrylic paint for glass costs a great deal more. I'd do samples first. Good luck!

There is a perm-enamel paint by Delta, that air cures. Folk Art also makes a glass paint as well that I think air cures.