Rancheros from old Baja used to (and some probably still) make whitewash paint from prickly pear cactus slime, lime (not the kind from trees, but calcium hydroxide) and salt. I found a recipe on the web a couple of days ago, marched out to the garden with my pruning saw and hacked up some paddles to soak. The water is supposed to get slimy--slime equals glue, I think. Maybe it's slimy. When I dip my hand in, the liquid comes off in an even stream, not in drips like water. I don't have any lime yet, and need more salt. Of course, I would never be satisfied with white, but plan to use red iron oxide powder, yellow ochre, or pick up some red clay by the roadside. Stay tuned.