I paint a monochrome underpainting in order to solve drawing and value problems on difficult subjects. The painting I am working on still is eighteen feet long with 11 figures and about two hundred roses, water, sky, drapery..... aaaarrrrgggghhhh. I couldn't imagine doing it full color from the beginning. To paraphrase an apocryphal quote from Van Dyck: do all you can at the beginning because there is plenty to do at the end.