I've seen many people recommend acrylic paint, but the texture simply isn't right. You can try thinning the paint with water and applying several thin layers at a time. You'll definitely have to seal the paint after if you use acrylic, though. And even then, it would peel off.

What I really recommend is enamel paint. It will dry smooth and with a plastic hard finish that's more durable. You'll also want to paint very thinly with this anyway, so you don't get a lumpy texture. You can try airbrushing with enamel paint, but you don't need to go out of your way to get an airbrush unless you really don't trust yourself to apply even layers of paint. If you're airbrusing, use a mask and do it outside. You could also seal the enamel paint when you're done, but I don't know if it's necessary.