An underpainting can also be applied like a wash, but rather than choosing a background, gradient, or blended color that you intend to see in the final painting, you’re choosing a complementary color that will filter through all other colors you apply on top of it. Artists use underpaintings because they can speed up the process in layering colors to look rich and vibrant, can serve as the medium tone in the painting, and can add interest to otherwise flat colors (Video 1).
For example, some artists who paint people apply a green layer before they start to paint the skin tones. Red and green are complementary colors on the color wheel. Having some of the green underneath the redder skin tones gives them more realistic depth. The opposite is true of plants. If you want to create a rich green leaf, consider adding a reddish brown underpainting before adding the green of the leaves.
EXPERT TIP: Always mix a little red into the shadows of leaves. It makes them much more compelling. Painting with greens straight out the paint tubes always looks flat and fake. Instead, play with complementary colors as underpaintings. For flowers, rather than mixing purple on the palette, try painting a layer of blue, letting it dry, and then painting a layer of red over it. Depending on the translucency of the petal, this may make the difference in whether the color looks realistic or not.