Hi Enzie,
I basically converted my value study to an underpainting like Karin would do. This was just a matter of lightening up the shadows and darkening down the lighter parts. Her post on the previous page of this thread shows what it looks like: basically a low contrast value study. It didn't take very long to do this since my drawing was already done in the value study. I let it dry overnight and then started painting in color.
There's a good example of underpainting going on now in the oil critique section, where Chuck Yokoda is doing a painting by this method.
I don't always do an underpainting, but I thought that since I had the grisaille already done, I might as well use it for that purpose. When I have used underpainting, I always did something like a full value study. Karin's method seems a lot more comfortable to me, because as you darken the shadows and lighten the highlights you see the right contrast level coming together as you approach it.
Thanks for the compliment on the color. I'm at work now, stealing the boss's time, but the image of my painting looks a lot warmer than on my CRT at home. Particularly on the shaded cheek. In my reproductions and in my painting, the darkest shadow on that cheek is a lot cooler than the rest of the shadow on that side. I actually kind of like the warmer version I'm seeing on the LCD screen of this laptop, but it's not very faithful to the actual painting. Hey, maybe my "old world look" is just an artifact of these computers!