I've been wanting to make my mom a hollow chocolate ball filled with truffles for her bday on feb. 9th, but I can't think of what to paint it with. I was thinking that because of the fat content in chocolate, that stuff might not stick to it. Any suggestions would be very helpful.

I did an experiment the other day, because I need to paint gold bits on a ceremonial Indian elephant chocolate transfer soon. I painted some white chocolate with gold lustre dust made into a paintable solution with vodka (just a few drops). It worked very well, and didn't come off when the vodka had evaporated - so my vote goes to lustre dusts! Hope that helps!

Cocoa butter and powdered colors. You may be able to find food grade cocoa butter at a pharmacy. I order mine from a cake supply.

You can paint with colored chocolate too.

Here is a link to a site that sells colors and blending powders for painting on chocolate
http://www.thesweetpalette.com/index.html