The idea of a watercolor under layer is to hint at the dimensions of color in the model. It is a gauzy sense of color, and its own interpretation of the model. A color sketch, it is far short of watercolor painting. It becomes a base I use when drawing the model with black walnut ink. The ink drawing is a separate, unblended expression of the model. In this way watercolor and ink work in tandem, each contributing a unique rendering of the model.
To learn how to order the materials described here, See the section under Resources on painting watercolor under layers.