The process takes recorded video (or digitally transferred film), a live video feed or computer output, and isolates and removes a single color in a narrowly defined region of the spectrum. The color is typically bright green or bright blue, because these hues differ so greatly from human skin tones and aren't usually found in clothing.
For the effect to work, green areas must be evenly lit and with no visible shadows, said Videomaker.com. Once green screens are identified and digitally removed, just about anything you imagine can be added back in, while the parts of the original image that aren't green remain unaffected. Chroma keying for live feeds requires hardware that can recognize and manipulate multiple video channels — layers defined by color — while recorded material can be changed in post production with video- or photo-editing software.