A green screen is a method for filming actors or objects against a green background to simplify the process of compositing them into a different scene. The green of the background staging is digitally removed and the new background inserted in its place.
The process of removing the background is called keying, and typically is produced by subtracting the red and blue channels from the green. Blue screens are also commonly used for keying, and occasionally red screens.
In recent years green screens have surpassed bluescreens in usage mainly because of the predominance of blue in clothing. It’s difficult to key blue clothing against a blue background.
In the past the process was incorrectly referred to as chroma keying. Chroma keying actually refers to an inferior chrominance keying method. To distinguish, modern visual effects artists refer to “bluescreening” and “greenscreening.”