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Chromakey, also known as Colour Separation Overlay (CSO) was traditionally done using a blue screen. In theory it could be done with any colour and these days it's usually green.
You shoot the performers against a blue (or green) backdrop. The electronics work out which bits of the scene are bue (or green) and substitute another picture in those areas.
The idea is simple, and dates back to the days of cine film but making it work well is difficult. You can often get a blue cast on the performer, or blue edging round the cutout and shadows usually look wrong. All these were overcome with more sophisticated circuitry, even back in the days of analogue processing.
Chromakey was more difficult if you only had the composite (PAL or NTSC) signal available. That gave a whole load of extra trouble and was widely disliked. Much better to use the RGB signals direct from the cameras.
It's all easier with a digital vision mixer. You've got adequately wideband colour difference signals available at all times. AFAIK green screen is still quite widely used.
Chromakey was often built into vision mixers (production switchers in US langauge). A common arrangement was to have a composite PAL or NTSC system with an auxilary RGB switcher to feed the chromakey circuit.
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