However, in most cases the stones are polished and difficult to paint. You may need to sand the stones, prime the stones, apply several coats of paint, and sometimes leave them outdoors for many months to "weather" them prior to painting.
An alternative to painting these stones is to engrave them first using an electric engraver tool (e.g., Dremel).
This video shows how to use this engraving tool on polished stones. In fact, engraving works best on the polished stones.
I've put this tool on my wish list so I can get even more creative with all the polished stones I've accumulated.