Chroma key is a technique for mixing two images or frames together, in which a color (or a small color range) from one image is removed (or made transparent), revealing another image behind it. This technique is also referred to as color keying, color-separation overlay, green-screen, and blue-screen. It is commonly used for weather forecast broadcasts, wherein 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 meteorologist stands in front of a bluescreen, and then different weather maps are added on those parts in the image where the color is blue. If the meteorologist himself wears blue clothes, his clothes will become replaced with the background video. This also works for green-screens, since blue and green are considered the colors least like skin tone. This technique is also used in the entertainment industry.

Source - Wikipedia

This video will go over the basics of chroma keying in using Final Cut Pro.

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Mike Colburn (DigitalMediaGuy)