RED - Siren (rising and falling note) Imminent danger of attack.
GREY - Siren (interrupted note of steady pitch) or Church bells (or, in Scotland, oral or whistle message)
Fall-out expected in an hour.
BLACK - Maroon, gong or whistle sounding a Morse ‘D’ – dash dot dot. Imminent danger of fall-out
ALL CLEAR – Siren (steady note) No threat of air attack or fall-out.