There is a watercolor medium called Ox Gall Liquid. Made from ... well never mind that part. Ox Gall improves the flow and acceptance of watercolor on the paper by decreasing the surface tension. When working wet into wet with two or more colors adding a few drops of Ox Gall to your water will allow the colors to blend and bleed smoothly into one another. It's great for doing really large washes, less chance for back runs.