We're going to do some real sugar skulls--and maybe even a very loose Day of the Dead altar if my energy holds out--later this month, but this was such an easy project that I couldn't help but try it as we "flew through" Mexico.
Our rocks were already pretty pale, so I didn't bother painting them white. As Maggy at Red Tent suggests, though, I did draw a rudimentary face onto each one to give the Kid some guidance. Then we just used regular, washable markers to make them bright and colorful.
If you're interested in Day of the Dead--or even just like the looks of these for Halloween--this is a great way to do skulls without having to fuss with molds and sugar and the like.