Color spill appears when key-screen light reflects onto a subject or when antialiasing, blur and transparency mix the key color with foreground pixels. Removing the solid background does not automatically remove those contaminated colors.
Despill correction reduces or replaces the unwanted hue, but aggressive correction can damage real colors and edge detail. Direct alpha export usually avoids chroma contamination because transparency is stored rather than inferred from a background color.
ELI5
Color spill is the unwanted green or blue tint that remains on a subject after a screen color is removed. The tint can be reflected light or a mixture inside soft edge pixels.
For example, a glowing animated mascot exported on green may keep a thin green halo after keying. An editor can reduce the tint, but a true alpha-channel render usually preserves the glow more cleanly.
