Chroma Keying: Keying is the process of isolating a single color or brightness value in an electronic image and using software to make that value transparent.
Why Green screen? In the visual effects world of Hollywood, blue screens are far more common than green. Green is the contrast from the natural skin tone and that is why green screens are used. Any clothing that matches the background too closely will also key out, punching a hole in your subject’s body, or making him/her invisible altogether.
Blue Screen: Blue screen was prodominently used in the 1900’s on films like The Thief Of Bagdad- People use blue screen over green screen because it is generally easier to chroma key, whereas people tend to use green screen because some of the people onscreen tend to wear blue and it is less expensive.