Bluescreenoptical effects or digital compositing.
Bluescreen is typically used for weather forecasts. The presenter appears to be standing in front of a large map, but in the studio it is actually a large blue background.
Petro Vlahos was awarded an Academy Award for his development of bluescreen techniques. His technique exploits the fact that most objects in real-world scenes have a colour whose blue colour component is similar in intensity to their green colour component. Zbig Rybczynski also contributed to bluescreen technology.
Other colours are sometimes used instead of blue, including green ("greenscreen"), orange, or grey. The choice of colour depends on the subject. Blue is normally used for people because human skin has very little blue colour to it. The same is also true for green, so the director can choose which colour to use depending on makeup and costume. Orange screens are often used with model photography where the model contains both blue and green components. Grey screens are beginning to be used in television for digital compositing. The grey colour is a very precise shade that a computer can pick up, allowing the subject to wear any costume or makeup.
Anecdotally it is said "green is preferable with older presenters as people tend to go bluer as they get older." - Anon TV news director.