Look at the local color of the skin tones of the figures below. Try copying the colors, keeping track of what you mix. In every case, I expect there was some of each of the primary colors involved. Experiment with allowing the component colors to remain incompletely mixed. You can start with a thorough mixture, where all the components have been mixed till they are evenly tinted throughout the wash, and then touch in a little of the components. If you're feeling brave, try double-loading your brush (loading more than one color at a a time). Do the same for some of the shadows.
This should be good practice for working from the model next week. If you have some large drawing paper (11x14" or larger), please bring a bunch for the short poses. It's fine if it's not watercolor paper, in fact, using relatively inexpensive paper encourages experimentation. Have a few pieces of the good stuff, too, for the longer poses toward the end of the session.