Chroma key describes when a scene is shot against a background of one colour. Computer software then makes the colour 'transparent' in the scene, revealing any layer (background) behind it. Green or blue are popular choices for chroma key as they do not usually show in human flesh tones. If a scene is being made using a chroma key, the colour of choice for the key should be avoided in the subjects or foreground otherwise those parts will become transparent too. Although this can be of use if you want part of your character to disappear!