The rocks in our yard are mostly large gravel pieces that were left behind from the construction equipment when our house was built. There are also quite a few man-made rocks - chunks of concrete with gravel in them. Once they have paint on them, you can't tell what they used to be!
The tempera paint is kind of gloppy, and I noticed that they didn't dry when we left them outside. Of course, Abigail kind of pours the paint on and then starts painting the newspaper and herself too.