False colour

Switch it on and skin, sky and shadow stop looking like a picture and start looking like a heat map. Each colour stands for a range of brightness values, and the ones that matter sit at the extremes: red for clipped highlights, purple or blue for crushed shadows, and a green or pink band marking correct exposure for skin. Set the key light until the face sits inside that band. Every camera uses a slightly different palette, so read the manufacturer's scale first.