My friend Tara asked to see some of these warm up images I do with my oil painting students and explain a little about the process. The goal with these is to concentrate on defining the objects mass, finding the full range of values and ignoring detail. You want to work quickly in order to train your eye to see the major shapes and light and dark relationships. In class we set up a very simple still life then each student has about twenty minutes and a limited palette of three colors to complete their study. This purple pepper (I don't think they really come in this color, but boy is it fun!) was painted by first staining the canvas with some quinacridone magenta and ultramarine blue and possibly a little burnt sienna. I paint a turp thinned layer down and rub off the excess paint with a rag or paper towel. Then I come back in with the darkest dark defining the objects shape and laying in the shadows. Next come the mid value color which is later wiped out to create the light areas.
The equivalent in water color would be to work wet in wet using non-staining colors so lifting could be achieved and because it's watercolor you'll be working light to dark.