Blue or green (and in the case of one of the Star Trek films orange) are used because they are the farthest from human skin tone (or in the case of orange allowed the overlay of mattes much more quickly, bring movie costs down). “Green is currently used as a backdrop more than any other color because image sensors in cameras are most sensitive to green. Therefore the green camera channel contains the least “noise” and can produce the cleanest key/matte/mask. Additionally, less light is needed to illuminate green because of the higher sensitivity to green in the image sensors. Blue was used before digital keying became commonplace because it was necessary for the optical process, but it needed more illumination than green.” (
http://en.wikipedia.org/wiki/Chroma_key ) So there you have it: green works better with the modern digital camera sensors and needs less light to work the way you want it to. Also, the green background is favored over blue for outdoors filming where the blue sky might appear in the frame and could accidentally be replaced in the process. You can actually use any color in this process, called Chroma-key, but what you need is a color that will not show up anywhere else in the shot so you don’t lose some part of the image that you would like to keep.