You can buy a kit at the local craft store of paint made for glass and ceramic (I think it's the Delta Cerma-coat) that has a dozen colors in little plastic pots on a stick. Good value if you need several colors. I prefer them over the larger bottles since I don't use them often and they tend to dry up.

I have also had good results using finger nail polish.

BTW, I learned a neat trick for painting dumbell eyes, place a few in the teeth of a large comb for painting. Easy to handle and you can get to both sides at one time.

I would take the time to top coat them with epoxy or 'Hard as Nails' for extra durability.

A smal finish nail or brad stuck in a scrap of wood makes a great tool for dotting the 'eyes'....