Chroma key

Why is blue or green background used in movies?

Chroma key compositing, or chroma keying, is a special effects in post production technique for compositing two images or videos together. The technique is mostly used in newscasting, motion picture and video games.

Why blue or green?
This technique can actually be done with backgrounds of any color that are uniform and distinct. Green and blue backgrounds are more commonly used because they differ the most from most human skin colors.

In case you are making a Superman shot try to not put him in a blue screen!