media Q19: It used to be Blue Screen. What is the difference?
Yes it did, and sometimes it still is. The background key colour is chosen so that it is the simplest thing to digitally remove. Film cameras were more sensitive to blue light, but modern video cameras find green easier. In both cases the choice of the colour comes from the fact that these colours are not present in human skin tones (unless your extremely off-colour!) and most often people are the chosen subjects. It is possible to use any colour as a background colour as long as it is not present in the spectrum of the thing you are trying to cut out.
