All colours in our visual range are made up of a combination of the three primary colours, red, blue, and green. In the bluescreen process, an actor or object is filmed against an evenly lit (i.e. entirely one colour) blue screen. In the compositing process, the blue element (i.e. the background screen) is removed via a colour separation process. The screen is blue in colour because blue is the smallest competent in the colour of human skin (i.e. skin colour has more red and green elements), so that when the blue colour is removed, it does not affect the appearance of the skin. This of course also means that the actor cannot wear certain blue clothing or the object cannot have blue parts.

With digital technology having almost completely replaced the traditional compositing processes, the colour of the background screen is becoming less important as greater accuracy in colour separation can be achieved with computers. In the television series Lois & Clarke: The New Adventures of Superman, the Man of Steel was filmed against a green screen for the flying shots to prevent his blue tights from disappearing into the composited background.

A chroma key subject must not wear clothing similar in color to the chroma key color(s) (unless intentional), because the clothing may be replaced with the background video. An example of intentional use of this is when an actor wears a blue covering over a part of his body to make it invisible in the final shot. This technique can be used to achieve an effect similar to that used in the Harry Potter films to create the effect of an invisibility cloak. The actor can also be filmed against a chroma key background and inserted into the background shot with a distortion effect in order to create a cloak that is marginally detectable.

In the 2002 film Spider-Man, in scenes where both Spider-Man and Green Goblin are in the air, Spider-Man had to be shot in front of the greenscreen and the Green Goblin had to be shot in front of a bluescreen, because Spider-Man wears a costume which is red and blue in color and the goblin wears a costume which is entirely green in color. If both were shot in front of same screen, one character would have been partially erased from the shot.

Blue is generally used for both weather maps and special effects because it is complementary to human skin tone. The use of blue is also tied to the fact that the blue emulsion layer of film has the finest crystals and thus good detail and minimal grain (in comparison to the red and green layers of the emulsion.) In the digital world, however green has become the favoured color because digital cameras retain more detail in the green channel and it requires less light than blue. Green not only has a higher luminance value than blue but also in early digital formats the green channel was sampled twice as often as the blue, making it easier to work with. The choice of color is up to the effects artists and the needs of the specific shot. In the past decade, the use of green has become dominant in film special effects. Also, the green background is favoured over blue for outdoors filming where the blue sky might appear in the frame and could accidentally be replaced in the process. Although green and blue are the most common, any color can be used. Red is usually avoided due to its prevalence in normal human skin pigments, but can be often used for objects and scenes which do not involve people.

Occasionally, a magenta background is used, as in some software applications where the magenta or fuchsia key value #FF00FF is sometimes referred to as "magic pink".

With better imaging and hardware, many companies are avoiding the confusion often experienced by weather presenters, who must otherwise watch themselves on a monitor to see the image shown behind them, by lightly projecting a copy of the background image onto the blue/green screen. This allows the presenter to accurately point and look at the map without referring to monitors.

A newer technique is to use a retro reflective curtain in the background, along with a ring of bright LED's around the camera lens. This requires no light to shine on the background other than the LED's, which use an extremely small amount of power and space unlike big stage lights, and require no rigging. This advance was made possible by the invention of practical blue LED's in the 1990s, which also allow for emerald green LED's.

There is also a form of color keying that uses light spectrum invisible to human eye. Called Thermo-Key, it uses infrared as the key color, which would not be replaced by background image during post processing.

(Source: wikipedia)