Colored lights produce complementary-colored shadows, and multiple colored light sources create multiple hued shadows

When an object is illuminated by a colored light in the presence of a second (white or complementary) light source, its shadow appears in the complementary of the colored illumination. Under red light in daylight, shadows are green; under green light, shadows are red; under yellow light, shadows are violet. This is additive mixing: where the colored light is blocked, only the complementary component of ‘daylight’ (or second source) reaches the surface. With two colored light sources, each casts a shadow of the other’s complementary, and the overlap area is black (no light). With three colored lights whose combination yields white (e.g., red-orange + green + blue-green), the shadows form a complete complementary triad and the fully lit area is white. The Impressionists controversially introduced blue shadows in painted sunlit scenes, representing this effect.

Examples

In Hydra: src(s0).mask(shape(4)).color(0,1,0).blend(src(s0).invert().mask(shape(4)).color(1,0,1)) approximates colored shadow in additive blend. In 3D rendering: colored area lights of complementary hue produce the shadow-color effect physically.

Assessment

Predict the shadow color under violet light in white daylight; explain what happens in the overlap zone when two complementary colored lights both cast shadows; why did Impressionists’ blue tree shadows scandalize viewers?