Colour-separation overlay
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Colour-separation overlay is the collective technical term for what is commonly called chroma keying.
It is a film or television graphical effect for compositing two full-frame images together, by discriminating the visual information into values of color and light. A particular colour is isolated by the system, used as a template or matte and then replaced with an alternate image or video.
It is most often used by weather presenters, virtual studios and special effects technicians.
The effect is accomplished by having the presenter stand in front of a solid-color backdrop onto which the image is projected. The typical use of the colours green and blue for the key have given the process the nicknames of greenscreen and bluescreen, respectively.