There is a medium you can
There is a medium you can add to powder frit to form a paste or paint like material. It is called Liquid stringer. I do not find it as nice to work with as oil paint (I also do art work). It does not flow from the brush like oil paint to canvas. But it does work for the kiln. Let what you paint dry thoroughly before you fire it in the kiln. Sometimes the end product comes out a little blurry. You can use multiple colors. There are also paints you can purchase that are specific to glass kiln work.