Some years back I'd seen a technique demonstrated involving charcoal dust. Water is applied with a brush to areas that are to remain whitest. Charcoal dust is then sprinkled across the page. Then a bucket of water is splashed across the page. The dust sticks to the areas that were dry. This process is best done outside because of the mess involved.
Afterwards I used spray fixative to keep the dust on the paper. On top of the dust I painted watercolors. I was pleased with the result.
This is the first one I did for my students to show them what was possible. Folks have told me it looks like a map. I simply painted complementary colors.
mixed media on paper, 17.5" x 24"