Underpainting is a foundational layer of paint in a work of art. It serves as the initial layer that provides the basic structure, values, and sometimes the color tonality of the painting. Underpainting can be done in a single color (usually a neutral tone like gray or brown) or a limited color palette. It is used to establish the composition and values before adding subsequent layers of paint. Artists often use underpainting to create depth and luminosity in their works, especially in oil painting.