I made a quick n'dirty video demonstrating how to get more realistic selection/watercolor interactions -- meant to more closely mimic the results of using liquid frisket in traditional watercolors:
http://youtu.be/RIhqZZHJxeg
Best,Jason.
Most of the Real Watercolor brushes in the default Painter library are not well suited to this look -- probably the only two I would recommend is "Grainy Wet" and "Wet Flowmap Fringe".
If you are not skilled at creating your own brushes, but still want to try this technique for yourself, here is a free brush I made:
Gradated Wash FM