Dear Joan:
There are many sound ways to paint a portrait. Modeling the head in combinations of raw umber and white is the underpainting technique Karin Wells uses. She gives a good demonstration at this link:
http://forum.portraitartist.com/show...0&pagenumber=1
I begin a portrait by creating a drawing in burnt umber. Burnt umber has more red in it than raw umber playing a role in the next step, the highing up process. When the underdrawing is completely dry, I begin building it up with titanium and zinc white and various mixtures of the two. The point is that when the zinc white is applied thinly over the dry, dark, burnt umber shadow areas of the underdrawing, it turns bluish-purple. Then when the underpainting is completed, you are in a position to apply warm transparent glazes over a neutralizing cool underpainting. The optical shadows produced are quite believable. The trick is to get the underpainting shadows as cool as possible before you glaze so that you don