The eye simultaneously generates the complementary of any color it sees

Simultaneous contrast is the phenomenon where a color induces its complementary in adjacent neutral areas — not as a physical property of those areas, but as a subjective sensation in the eye. A gray square on a red background appears greenish; on a green background it appears reddish. The eye demands visual equilibrium and generates the missing complement spontaneously. This effect: (a) is subjective — not capturable by a camera in the original neutral; (b) intensifies with sustained viewing and greater luminosity; (c) can be suppressed by adding the complementary explicitly (making the simultaneous shift ‘use up’ its demand), or by introducing light-dark contrast which weakens simultaneous effects; (d) affects all color pairs that are not precisely complementary — they shift each other toward their own complements. Practical consequence: any solid-color background will alter the apparent hue of overlaid elements.

Examples

In Hydra: solid(0.5,0,0).layer(shape(4,0.3).color(0.5,0.5,0.5)) — the gray square will appear to shift greenish due to the red background. To cancel: add enough green tint to the gray, or make it slightly brownish-orange.

Assessment

Design an experiment to demonstrate simultaneous contrast with code; explain why adding the complementary color to the gray neutralizes the effect; predict which gray shifts most on a violet vs. an orange background.