Air Raid Warning Message Colours

The air raid histories written shortly after the war include a wealth of information about the running of the Civil Defence Services. I recently obtained a copy of the book covering events in the city of Gloucester. It's full of detail but this short piece about air raid warning messages and colours is most interesting.

"The siren was operated immediately the "Action Warning" message was received and this was timed to give not less than five minutes warning to that part of the district nearest the raiders’ line of approach. Some departments, however, e.g. A.R.P. organisations, both local and industrial, needed a longer interval than five minutes in which to make necessary preparations; to these an earlier message, called the Preliminary Caution, was sent, possibly about ten minutes before the Action Warning. This was kept confidential and was never divulged to the public. Other messages in the early days are given below: 

Preliminary Caution; was Air Raid message YELLOW 
Action Warning; was Air Raid message RED
Raiders Passed; was Air Raid message GREEN
Cancel Caution; was Air Raid message WHITE

and most A.R.P. Headquarters, Posts and Depots devised a system of coloured lights or cardboard discs to correspond, so that all present could easily see the situation. 

In July 1940 an additional message, “Air Raid Message PURPLE” was introduced; issued only during the hours of blackout, this was a ‘lights’ warning, and on its receipt, all exempted lighting in docks, factories, transport undertakings, etc., had to be extinguished, and the Police might direct motorists to extinguish head lights. At the same time "Air Raid Message GREEN" was abolished, the WHITE serving the purpose of cancelling all messages which preceded it. In October 1941 the "Air Raid Message YELLOW" was abolished leaving only PURPLE, RED and WHITE, and these three remained throughout, except for certain modifications in 1944 to meet the new situation caused by flying bomb attacks. 

At the outset and for some considerable time thereafter, the messages were originated by Fighter Command, based on information received by them from the Royal Observer Corps. Finally, warnings were initiated direct by local warning officers of the R.O.C."

City of Gloucester Civil Defence history