A chroma key is the removal of a color (or small color range) from one image to reveal another "behind" it. The removed color becomes transparent. This technique is also referred to as color keying, colour separation overlay, greenscreen and bluescreen. It is typically used for weather forecasts. The presenter appears to be standing in front of a large map, but in the studio it is actually a large blue or green background.

The principal subject is photographed/filmed against a background having a single color or a relatively narrow range of colors, usually in the blue or green. When the phase of the chroma signal corresponds to the preprogrammed state or states associated with the background color(s) behind the principal subject, the signal from the alternate background is inserted in the composite signal and presented at the output. When the phase of the chroma signal deviates from that associated with the background color(s) behind the principal subject, video associated with the principal subject is presented at the output.

The best known example is television weather broadcasts, where the meteorologist is filmed in front of a flat, even colored green or blue screen. The background color is removed electronically, and replaced with a weather map which the meteorologist points to (by glancing at monitors hidden off_camera). The meteorologist must not wear clothing with the background color, or else part of the clothing will be replaced with the background video.

With better imaging and hardware many companies are avoiding the confusion often experienced by weather presenters by lightly projecting a copy of the Colour-separation overlay (CSO) image onto the CSO blue/green background. This allows the presenter to accurately point and look at the map without referring to the monitors.

The most difficult part to setup a blue- or greenscreen is the even lighting and the avoidance of shadow, because as narrow as possible range of blue (or green or other) color is made transparent. A shadow would change that to a darker color and it will be opaque.

See also: luma key, matte key, bluescreen, greenscreen, composite video, chrominance signal, Federal Standard 1037C