Yes you can use acrylic artists paints on leather which IMHO are better than Tandy's Cova colours.
Coloured markers, like sharpies also work well for outline work. Definitely seal the surface with a clear acrylic spray like this Fiebings one.
http://www.tandyleatherfactory.com/en-usd/home/department/leather-dye/leather-finish/2205-00.aspx
There are a couple threads on what paints to use in here if you search acrylic paint or painting leather.
Hope that helps, Scott