Ok, silly me. I don't know why I sought out what was best to use. The minute they opened them they wanted to decorate them NOW! Should've seen that coming.
The only thing I could find was Easter egg dye. Worked beautifully on the hair and sort of worked on the body. Grabbed some permanent markers at the grocery store and those went on just fine. They're very happy with the results and since the ponies were only $3.99, I'm happy that they're happy.