Hello- when I am instructing a painting class I do spent one or two sessions on just underpainting. I believe a well developed underpainting only enhances the final painting.
Here is an example of how I approach underpainting using watercolors. I use water color as it does not alter the surface of the uart paper . I use the 400 grit.
As you can see I have laid out he composition and was able to settle on the color scheme well before I apply any pastel. i have tried simply splashing water color on the pastel surface in a random fashion , however I feel I am able to focus more on the subject and composition when I stay with the design of the piece . As with pastels, when applying watercolor, I start with the dark areas first to create a vlaue study. I then add the local colors ie: yellow and rusts.