Underpainting is the foundation of a painting. In this first layer I like to use complimentary colors to suggest mood, movement and contrast. Complimentary colors are those opposite each other in the color wheel: red/green, yellow/violet, orange/blue. For example, in the final piece the color of tree leaves are hues of green. In the underpainting, I will use pink or red in these areas. I like to paint this layer with general shapes and broad strokes of color without any detail of the final piece. In areas of the painting I do not what to emphasize, I will not under paint.
Why bother at all if the underpainting will be covered anyway?
Paint is generally a thin layer which can be sheer in some areas of a painting. As layers of paint are added, some areas beneath show through and the eye of the viewer picks up on the subtle differences in tone, giving the painting more dimension. You can also try putting complimentary colors directly next to each other to achieve a similar effect.