The background footage was shot first and the actor or model was filmed against a bluescreen carrying out their actions. To simply place the foreground shot over the background shot would create a ghostly image over a blue-tinged background. The actor or model must be separated from the background and placed into a specially-made “hole” in the background footage.

The bluescreen shot was first rephotographed through a blue filter so that only the background is exposed. A special film is used that creates a black and white negative image - a black background with a man/spaceship shaped hole in the middle. This is called a female matte.

The bluescreen shot was then rephotographed, this time through a red and green filter so that only the foreground image was cast on film, creating a black sillohuette on an unexposed (clear) background. This is called a male matte.

The background image is then rephotographed through the male matte, and the bluescreen shot rephotographed through the female matte. An optical printer with two projectors, a film camera and a “beam splitter” combines the images together one frame at a time. This part of the process must be very carefully controlled to ensure the absence of “black lines”.

Yellow screens were used in the Sodium Vapour Process developed by Disney for the film Song of the South.

There are some modern screens that at first sight appear grey, but are in fact coated with tiny half-silvered glass beads to give a significant degree of retroreflectivity. A ring of coloured lights (usually LEDs) is placed around the camera lens, and the screen reflects this colour back to the camera. This technique reduces problems from performers casting shadows on the screen, and allows operation at low lighting levels. As the screen colour is defined by the colour of the ring light, it is easier to change the screen colour quickly, and to use a colour with a narrow range, making it easier to distinguish between the colour of the screen and colours on the subject.

Other colours are sometimes used instead of blue, including magenta (The Matrix), yellow (some 1970s episodes of Doctor Who), orange (Apollo 13) and red (Air Force One). The choice of colour depends on the subject and specific technique used. Blue is normally used for people because human skin has very little blue colour to it. Green is used because digital camera retain more detail in the green color channel and it requires less light. Magenta screens are often used with model photography where the model contains both blue and green components.