Underpainting is an initial layer of paint on the canvas that will serve as a base for the entire painting. There's a popular misconception that underpainting has to be monochromatic, but underpainting in color allows the artist to easily control the overall tone and mood of the painting. Choose the underpainting color and load it as your Foreground color. Now go to the Edit menu and choose Fill, in the dialog choose Foreground from the Use menu and click OK. Add a new layer above this one to begin your painting. Now choose your Brush and be sure to turn down the brush's Opacity in the Options Bar to let the mood of your painting shine through.