The main effect that a Chroma key screen has is brightness. Chroma key screens are typically colored in a brilliant green or blue color. The brightness of the screen is very obvious, and will make the subject of the video much more noticeable than they would be with a dark-colored screen or a backdrop with multiple colors on it.
A Chroma key screen gives an effect of clear subject definition. Any person or object who is placed in front of a Chroma key screen will stand out in very sharp relief, which makes it easier for the technicians working on the film to insert backgrounds later.
Chroma key has the effect of making human skin tones stand out dramatically. This is due in part to the fact that green and blue colors tend to provide the greatest contrast with most skin tones. "Blue was a popular choice in the early days of color motion pictures, because it is complementary to the reds found in human skin. Green became popular because digital editing systems can isolate green with less light in the background," says the TV Tropes website in its Chroma key history.
Chroma key screens are sometimes used to create an "invisibility" effect. This effect can be achieved when a subject is clothed in the same shade as the background screen. Actors wear a body suit that is the same color as the Chroma key screen, rendering that portion of the body "invisible" when seen in post-production.