WW2 Air Raid Warning and Message Colours - Yellow, Red, Green & White

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

WW2 Air Raid Warning and Message Colours - Yellow, Red, Green & White

Air Raid Message Yellow and Air Raid Warning Red

Air Raid Messages Green and White