As someone who used to use nailpolish a lot (on my nails), I'd advise against using it on keycaps. Nailpolish is not made to be durable like oil-based paints for cars and such. It will wear off fairly quickly with use, as it wears off nails. I would advise against using normal oil-based spray paint as well, because, while durable, it will feel sticky. Instead I would look into getting white / light keys and dying them with something like RIT (mixed results there too, from what I've read). Your safest bet is to spray paint with oil enamel if you don't mind the sticky feel or perhaps try Plasti-dip. Or just buy keycaps already made in colors you like.