One principle in color theory is that layering basic colors close to another so that they are visually mixed by the eye leads to a richer result than mixing the colors together as a solid block of pigment; for example, lightly sketching red, blue, and yellow together to create the impression of brown rather than only using a brown pencil. Artists are encouraged to use this principle by carefully observing the color of the subject’s undertones and shadows to bring depth and life to the picture instead of just using black or white to darken or lighten a picture. The sister to this principle is the one of complementary colors: the color of the shadow will usually be opposite on the color wheel from the color of the highlight. A scene with blue shadows, for example, will have orange tones in the highlights. Usually the shadows will be dominated by a cool color, but warm shadows can lead to interesting results!
Here is a quick sketch of some black-eyed-susans. Each one uses a different color for the shadow and highlight undertones. Clockwise from left are purple and yellow, blue and orange, green and pink, and deep red with light green. Which result is your favorite? Tell me in the comments!