Bluescreen for movie CGI backdrops

[hewo]

Why do they specifically use blue (or green) for overlaying CGI?

[codytesnow]

There are a couple of reasons. First, this method is called "chroma-keying". There is also "alpha-keying" and "matte-keying". Originally, black velvet was used, then the film would not be exposed except for the object in frame. Bright blue or bright green is used because 1) these are not colors found very often in nature, so, when shooting an object, that color, not found on the object, would be missing, and therefore transparent. For instance, if you used a red screen, all types of skin tone would become transparent because they are all base red. 2) Chroma-keying was first used in video, RGB. After discluding red, as shown above, green and blue were the only other colors you could drop from the signal.