why do they use blue screen in movies or green? why not red or white?

Answer 1

They choose monochromatic light so they can filter out that single wavelength. If they filtered out white, they would filter out everything. If some artifact of the screen doesn't get filtered out, it's much more noticeable as red than blue or green with the most commonly used subjects. Also, as you can see from the reference, the human eye is least sensitive around 475nm, making that an especially good choice.