Chroma key

The keyer looks for one colour across the frame and makes it transparent. Green is standard because sensors capture more green information and it sits furthest from most skin tones, while blue works better if the subject is wearing green. Almost every bad key comes down to lighting, and no software setting fixes it. The screen has to be lit evenly and separately from the subject, and the subject has to stand well clear of it, otherwise green light bounces onto shoulders and hair and produces spill that no amount of tolerance-slider fiddling will clean up. Fine hair and motion blur are the other two giveaways.