Spill

Color spill occurs when the light reflected from a colored background — typically a green screen or blue screen used for chroma keying — bounces back onto the foreground subject, causing their edges, hair, and any adjacent surfaces to take on the background's color cast. When a subject stands in front of a lit green screen, some of the green light reflected from the screen hits the subject from behind and the sides, casting a green tinge on the edges of their outline, the tips of their hair, and any translucent or reflective surfaces. This contamination makes chroma keying more difficult, because the keying algorithm needs to remove the background color without also removing the subject's spill-contaminated edges.

Spill occurs for a simple physical reason: green screens must be evenly lit to key cleanly, and lighting a large colored surface inevitably results in some of that colored light bouncing back into the scene. The intensity of spill depends on the screen's brightness (brighter screens produce more reflected light), the subject's distance from the screen (closer subjects receive more reflected light), and the subject's surface properties (light-colored clothing and fair skin pick up more spill than dark clothing). The combination of standing too close to an under-separated, brightly lit green screen while wearing white clothing is the worst-case scenario for spill.

Managing spill involves both production and post-production strategies. In production: maximize the distance between subject and screen (at least 1–2 meters), avoid lighting the screen brighter than necessary, and if possible add a complementary-colored kicker light (slightly magenta for green screen) on the subject's back-edge to counterbalance the green reflection. In post-production: use "spill suppression" or "despill" tools built into chroma keying plugins (After Effects Keylight, DaVinci Resolve's Fusion, Primatte) that selectively reduce the green saturation in the subject's edge areas without affecting the overall key. A clean, well-lit green screen composite requires both disciplines working together; relying solely on despill to fix excessive production spill rarely produces professional results.