I took the underpainting back to Karl's class and we glazed it--essentially we mixed up a dirty purple-blue color and did a transparent wash over the whole painting using a lot of medium. Then I went in with specific colors and painted over both the underpainting and the glaze coat.
Karl says glazing often helps the colors look richer and more lifelike because of the many layers. Apparently some of the old masters used to glaze their paintings more than once.
I wanted to keep a semi-monochromatic look to the painting, but add a bit of color here and there. I think it's my most successful portrait in oils so far. Having that underpainting done sure simplified things when I got to the color stage (although that was, in itself, tortuous for me, even though Karl thinks I'm "getting it")