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The Colour Strike was an industrial action by technicians at all ITV companies from 13 November 1970 to 8 February 1971 (although some shows made during this period in black and white were having their first transmission as late as December 1971) who, due to a pay dispute with their management, refused to work with colour television equipment.
At that time ITV had recently switched to colour transmissions, requiring the individual companies to invest heavily in new equipment. Early colour television studio cameras consisted of four tubes to relay the picture: three were receptive to colour (red, green and blue - the chrominance signal) with the fourth providing a high-resolution monochrome image (the luminance signal) which was still required as many viewers still watched on monochrome receivers. The final colour picture was created by combining the chrominance and luminance signals, but the technicians simply switched off the colour tubes whilst this dispute took place.
This meant that even though colour equipment was available, all shows were recorded and broadcast in monochrome, thus denying the ITV companies the ability to sell airtime at the higher value that colour transmissions dictated.
In some film sequences for location shots in these programmes (shot in colour), the colour signal from the telecine machine had to be switched off in the vision mixing desk before being recorded to tape, but this was partly unsuccessful, leading to some film sequences being recorded with an odd array of pale colours (as for items where the colour is a mix of two primary colours, only one primary colour would show). This is prominent in the second series of Hadleigh, for example.
The first Coronation Street to be broadcast in colour was on 3 November 1969, but due to the strike, many 1970/71 editions (such as the death of Valerie Barlow by electrocution) were shot in black and white. The last monochrome edition was shown on 10 February 1971.
There was also a short dispute two years later in early 1973 affecting both BBC channels and ITV as well.

From Wiki.