The Green Screen:
The green screen, or 'Chroma Key', is single coloured back drop which can be used to add a full background or fragments of a setting in a media product via post-production.
The scene is recorded as normal but elements are later superimposed using photoshop software through chromakeying, which is identifying one colour in the background (this will usually be green) and making this colour transparent.
..Because of the absolute removal of one particular colour it is important for all subjects in the scene to steer clear of this colour; otherwise unwanted fragments will be removed and situations of this sort can arise...
If the chroma key is used to add a realistic background to create a sense of realism in the scene, it is particularly important to match the lighting correctly; if the background is exterior lighting of the subject should either be natural or artificial but made o look natural etc.