Chroma keying deletes pixels by color: shoot against saturated green, key the green out, and the subject floats on whatever background the mix provides. Green won because sensors resolve it cleanly and skin contains little of it.
How keying works
A keyer examines each pixel and scores its distance from the target color. Pixels close to pure screen green become fully transparent, pixels far from it stay opaque, and the narrow band between becomes partial transparency, which is where hair, motion blur, and glass either survive or die. Modern keyers add spill suppression (desaturating the green light that bounced onto the subject) and edge choking to tighten the matte. The quality ceiling is set at the shoot, and lighting decides it: an evenly lit screen keys in one click, a wrinkled or shadowed one costs an evening.
Green screen or blue screen
Blue ruled the film era: film stock resolved blue finely and blue paint behaved under tungsten light. Digital sensors flipped the advantage, since their Bayer pattern carries twice as many green photosites, giving green the cleanest channel. Blue remains the pick when the subject wears green or when a night scene benefits from blue spill instead of green. The principle is constant: key on the channel your subject contains least.
Keying in live rigs
Live rigs key in real time: dancers over generative backgrounds, a keyed camera feed as a VJ layer, streamers floating over gameplay. The keyed result is transparency, which in digital terms means an alpha channel: chroma key manufactures alpha from color, where formats with true alpha carry it natively. Between apps on one machine, Syphon and Spout pass that alpha directly, no green required.