I don't know a direct solution off the top of my head.
As a workaround, you could fill flats in white (or whatever your page is) on one layer per color that you want, then add a clipping layer on top so it would be super quick to use the brush over them.
Sounds like a pain but if you keep a folder with layers all set up it wouldn't take a terrible amount of time.
Another option is to fill with flat colors and add a texture layer on top!