Here is a painting of a marsh in Lillie Park, near Ann Arbor, MI. What I found impressive was the blues, purples and greens of the water (reflecting the clear blue sky) and the marsh grasses.
I chose to do a watercolor underpainting to convey the basic hues as well as to add a sense of atmosphere to the landscape. The paper is Uart 400 grit.
Here is the reference photo:
I started with purple watercolor applied with a foam brush:
I then began to apply greens and blues:
Here is the final watercolor underpainting. The watercolor is simply from a pan set brushed on the Uart paper with a foam brush. You can also use a bristle brush. The purpose of this underpainting is to capture the basic hues. In addition, the watercolor can save some pastel application when you allow it to show through the pastel painting. Other varieties of underpainting include capturing value and (for the adventurous) complementary colors. You can also accomplish an underpainting by applying some pastel to the sanded paper then diluting with alcohol by applying with a foam or bristle brush.
Several of my paintings were begun with an underpainting. Here is the final version of "Birch Ballet," which was begun with a watercolor underpainting.
You can review my blog on Birch Ballet and see the underpainting established in the early stages of the painting.