Cova Color acrylics work beautifully on leather and fabrics. Just paint them on, then finish up with a sheen product to seal it in and make it look a little shinier. I've also had good luck with the cheap acrylics you can find at places like Wal-Mart for painting on leather for non-wearables. The cheaper acrylics tend to chip, but if you seal it with something, it might not be as bad.
Cova Colors and Super Sheen are available at Tandy Leather