Definition:
The initial or lowest layers of paint put down in a painting before the detail of the painting is put down. Some artists use underpainting to establish tonal values in a painting, effectively painting a monochrome version of the final painting to get all the tones right before adding color. Others use underpainting to establish areas of color as a first step in building up colors through glazes.
Underpainting doesn't need to be done in gray, it can be done in any color(s). It can be very effective to create a cool or warm underpainting as a contrast to warm or cool colors in the final painting, letting the underpainting show through in small areas.
