Green Screen’ is also known as Chromakey. Keying is the process of isolating a single colour and brightness and isolating that from the rest of a scene in post-production.
Colour keying identifies a particular colour to remove. In the early days of the technology, this tended to be chromakey blue, but chromakey green was found to be a more useful colour, not least because it occurs less often in sets and costumes.
It also requires less light to correctly illuminate. The colour green is the furthest colour from skin tones.