What do you call the green backgrounds which are used to create CGI backgrounds, weather reports etc.
Why is green color always used?
Chroma key is the effect used to replace a single color with video from another source. Commonly called "green screen", other colors, originally blue, and now orange or magenta can also be used, depending on the subject and effect desired. Wikipedia has a fairly detailed description.
These are called "green screens" because of their green appearance. Green is used because it's one of the colors lesser worn on people. For example, they used to have blue screens, but if the person was wearing blue jeans then their legs would disappear instead of just the background.
As Jim Green mentioned there are other colored screens as well, probably to use in different situations depending on what people are wearing.