More from “Light for the Artist” by Ted Seth Jacobs:
The Inseparability of Value and Hue. Many painters, such as the surrealists, for example, lighten values only by adding white. They treat the value scale as if it were only a lightening and darkening of the same hue. This is a serious mistake for the optical artist. It is an essentially “black-and-white” approach. The result is similar to a colored drawing, with local colors washed over the value changes, and does not take into account the fact that the light is colored. Value and color change together, organically. We cannot run up and down the value scale without constantly varying the hue.
For example, if the light source has some kind of (unnameable!) yellowish coloration and the shadow turns correspondingly complementary, as the values darken the yellowness will also drop. We need to incorporate this hue change into each value change. We must see each change as a colored value. Otherwise we are essentially painting in a monochrome. We also must avoid “tinting” value changes with the same color. For example, if the light itself is yellowish, we ought not to put the same intensity of yellow everywhere in the light.Also take care not to give the shadow the same kind of hue as the light. The color of the shadow can be deceptive. For example, when the body is under a yellowish light, the reflected light in the shadow may be very warm. However, approaching the terminator, where there is the least influence of reflected light, the shadow may show more of its complementary nature. Some students notice only the very warm reflected lights and paint all the shadow warm. This makes for a warm-on-warm effect that does not correspond with optical reality. The effect is rather heavy, or hot, and the picture will not have the feeling of light speeding through it. The hue will be turgid if it is too similar in the and the shadow.