I paint on my windows every year. All you need to use is any tempera with a
little dish soap (like Dawn) mixed into the paint. There is no magic formula,
just few squirts per cup will do. When you are ready to remove the paint,
just sponge it off. You may need to lightly scrape a few stubborn spots.
However, the windows will be as clean as a whistle when you are done.
You can see one example of an early window we did. (Since then, we cover
almost the entire window) and my truck which I let the kids paint.
Go to this web page and scroll down to the bottom set of pics.