Today I've been forcing some underpainting stuff for highlights in both figures. Look closely and you'll see that most of the highlights are either a straight white or a light tan pattern paper. Also in some of the darks I begin establishing them by using a few large strokes of dark gradiated colours. Once in, I work around them by using the next lightest tone to model the figure and focus on trying to maintain that same intial stroke with as little rework as possible. I haven't gotten the effect as well as I would like but as I continue this work I'm sure I'll become much more confident in this method.