What can you use to paint over enamel paint on the walls?

You can use either enamel (oil-based) paint or latex (water-based) paint. If you are using latex paint, you need to use a primer (undercoat) first. The paint store will have a special primer that allows you to paint latex over enamel. If you are going to do this, it is also a good idea to either sand the enamel paint before you paint over it, or wash it with TSP (trisodium phosphate) cleaner to remove the gloss.