If you want to play with fire SWMBO nail varnish is just as good, but if you get caught make sure you have an answer.
Acrylic paint are good, the local £1 shop usually sels starter artist kits that are good and simple..
All of the above has been mentioned works, but I use Epoxy and have be known to mix a little acrylic paint in to the mix just to make sure the colours don't get scratched on inserting the blank..
Trial and error to find what you are happy with.