I told the kids to clear away the new snow and paint on the old, hard, frozen, dirty snow. It didn't seem to help.
I probably won't attempt this again with my class. Paint got everywhere, and even though it was washable, I felt bad sending it home that way. So I scrubbed a bunch of mittens and hats and boots in the sink when we got back. Spray bottles might be a better option. But after seeing those paintings at Happy Hooligans, how could I resist trying it myself?