Have you seen the Sugar Skull Mold Kit from Cloth Paper Scissors?
I used the products in the kit to create this colorful calaveras: an Oaxaca plastic mold, Creative Paperclay, Tulip paints, and flowers are all part of the kit.
You can purchase it here.
The paints come in bottles with nice, fine tips, making it easy to draw any shape you like. Add dots, lines, free-hand flowers . . . whatever you like.
Use extras from your stash, too. I used a black Sharpie to trace the eye sockets and the teeth.
Have fun with it! I know I did.