What does the technique 'underpainting' typically refer to in oil painting?

Underpainting refers to the process of applying a base layer of color to the canvas before the final layers of paint. This foundational layer is often created with a monochromatic color or a limited palette and serves multiple purposes. It provides a uniform surface for subsequent layers and can also help establish the composition, values, and overall tone of the painting.

An underpainting can enhance the richness of colors applied on top, as it adds depth and complexity to the finished artwork. This technique has been used historically by many artists to create a structured approach to painting, allowing for greater control over the final appearance of the piece.

The other options do not accurately describe underpainting. While cleaning brushes is essential for maintaining tools in good condition, it does not pertain to the technique of underpainting. Similarly, mixing colors is a fundamental practice in painting, but it is not a specific method associated with underpainting. Lastly, outlining the subject is a different aspect of composition and drawing, which is not the focus of underpainting.