This confidential early-war memo issued to Air Raid Wardens covered the various coloured warnings that were used during the second world war:
Air Raid Message - Yellow: wardens to gather at their posts (public not informed)
Air Raid Warning - Red: air raid imminent - wardens dash out into their their sectors blowing short blasts on their whistles
Air Raid Message - Green: raiders passed - wardens walk their sectors blowing long blasts on their whistle
Air Raid Message - White: All clear sent to wardens' posts (public would be informed by 'All Clear' siren)
Later in the war the 'Air Raid Message - Green' was abolished. A separate 'Air Raid Message - Purple' was introduced
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