Chroma key

Shooting against an even colour so it can be replaced later. The green screen: a promise that the background is not yet real.

Chroma key is a deferral: shoot the subject against one even, unmistakable colour, and postpone the world around them. Green, because it sits farthest from skin, and even, because the removal is only as clean as the colour's consistency. The screen is a promise that reality will be added later.

The promise is kept or broken at the edges: hair, motion blur, glass, the green light bouncing back onto shoulders. Clean keys are lit, not fixed, flat screen, separated subject, matched direction between subject and the world that will replace the colour. A good key looks like nothing; a bad one converts the entire shot into an apology.