Watercolor is a great medium for getting expressive with colors and marks. The medium's ability to produce layers of translucent colors lends itself to the possibility of deep saturation of colors. Many artists have mastered the ability to be deliberate with their brush strokes and make bold bursts of color that lead to incredible expressive paintings.
Some of us struggle with this ability to be expressive with colors. We are envious of the bold strokes and splashes of strong color. For those of us in this camp, we can simply rely on our under painting to push our expressive shortcomings.
The underpainting is simply what it sounds like. It is a painting under the finished painting. Many painters simply skip creating an under painting, forgetting that an underpainting can make the finished painting more successful.
In most cases, an under painting's purpose is to define the shadows and highlights in a painting. Doing this allows the artist to establish a value range so that the focus can then shift to the color.
But an underpainting can establish expressive color as well.