Correct me if I'm wrong, but I haven't found a way to "erase" in a watercolor manner -- that is, to be able to paint with a sort of subtractive paint, so it removes color rather than adding it. What I have in mind would be like erasing with flow, or painting with bleach. Does Rebelle already have this sort of feature and I just haven't figured out how to do it? I know it doesn't exist with real-world media but it would be a useful tool in the simulation.