What kind of paint do to use to paint plastic sunglasses?

Do a search for "plastic paint" and you will find several manufacturers of products that should allow you to recolor your sunglass frames. One of the more popular brand is Krylon Fusion, but many paint manufacturer have their own variation. There is not a huge choice of colors, but all the primaries and a few shades are covered so you should be able to find a color that suits your needs.

FYI, "plastic paint" is not quite like traditional painting. It is actually more accurately called "plastic dye" because you are actually deposition pigment into the surface of the plastic. Think of it as a special form of colored paint mixed in with a solvent that slightly melts the surface of the plastic, allowing the special paint pigment to molecularly bond with the surface. This is why "plastic paint" does not chip like traditional paint.