If the color temperature of a fill light tends toward a certain hue, our visual system compensates by making the key light appear to be composed of the complementary color.
On a camera, this is the result of “automatic white balance.” Scientists call it “chromatic adaptation.” Artists call it “simultaneous contrast” or “induced color.” These are all related ideas, and they’re fun to experiment with.
I set up a little plaster head over a green-colored table surface, which bounced the fill light into all the shadows. The greenish shadows made the light side appear reddish or warm. I painted the study with strings of color based on viridian for the shadows and venetian red for the light. Note that the bounced light within the keylit side (like under the brow) is warmer than the shadow side.
Then I took my little plaster horse head and set him up over a reddish source of reflected light. Now the same key light appeared to shift toward a more greenish hue.
I set up this maquette with a neutral surroundings to eliminate the chromatic contrast between the light side and shadow side, and painted it with white and umbers. This is more the standard practice in atelier training, but it's a real eye-opener to explore color effects along with tonal modeling, even with ultra-limited palettes, as I've been using on all these studies.
Leave your casts or maquettes white if you’re going to be observing them directly with the eye, because these color effects are more pronounced. Paint them a 30% gray (flat spray primer works well) if you’re going to photograph them, because otherwise the bright tones will burn out on the top end.
Here’s a concept sketch for a science fiction paperback cover. Cool light with warm shadows isn’t used as often as “golden hour” lighting. It conveys a weird, artificial feeling, because only unnatural environments have cool, direct light coming from below.
For a fun optical illusions demonstrating chromatic adaptation, link
and Duke University's Dale Purves "see for yourself" lab, link.
Related GurneyJourney posts:
Colored Light and Form, link.
Golden Hour lighting, link.
Warm and Cool Colors, link.
Key and Fill Light, link.
Character Maquettes, link.