Chroma key
Making one specific colour transparent so another image can sit behind it — greenscreen, in everyday terms.
Green and blue are used because they sit furthest from human skin tones, which makes the boundary between subject and background simple to compute. Green dominates in digital production because camera sensors have the most green photodiodes and therefore the most information in that channel.
Two problems recur. Spill is green light bouncing off the screen and tinting the subject's edges, requiring its own correction. And heavy chroma subsampling destroys exactly the colour edges keying depends on, which is why professional production shoots in 4:2:2 or 4:4:4 rather than the consumer 4:2:0.