What is chromakeying? A basic answer is having talent stand in front of a green screen and then use a device (hardware or software) to remove that color and replace it with another image or video source. This gives the illusion of the person being somewhere else, and it's a more common technique than you might realize. Not only is the weather man being keyed in front of a green screen, but more and more often field correspondents and even newscasters are green screened into virtual sets. Sometimes green screen is also called chromakeying, more general term which covers keying on many colors including blue and red. Green screen is commonly used when control of what the talent wears is out of the producers hands (in a field environment perhaps), because people tend to wear more chrominant blue than green making green screen ideal for mobile chroma key. Green screen tends to yield a less attractive spill, because green screen backdrops reflect more candela the spill off of a green screen backdrop will be brighter and more noticeable than a blue screen backdrop.

Getting a good even color behind the talent is the first step in green screen, this usually requires a chromakey backdrop in the form of cloth or paint. Some green screen paint can be picked up at hardware stores or lighting places like Studio Depot (Burbank, Ca), blue cloth can also be found many places. Some companies sell green screen pop up green screen backdrops which travel easily and popup like a windshield shade.
Another alternative is Chromatte, a unique retroreflective material which is gray to the eye, requires little or no light, and casts no spill on the talent because the material is gray to the naked eye. But to the camera it sees it as a perfect blue or green. This is because of the Litering, a ring of LEDs around the lens which casts blue or green light onto the Chromatte material. Chromatte is also very portable in the form of a Chromaflex, a 7x7 popup which folds down into a 3 foot hoop. Chromatte is superior to Green Screen in many ways.