Code Red was a military code that suggested a dramatic emergency. UNIT issued Code Red Sontaran when the Sontarans invaded the Earth in 2009. (TV: The Poison Sky)
UNIT Control in Geneva sent out a wordwide Ultimate Code Red when the Daleks invaded the planet later on. (TV: The Stolen Earth)
Captain Erisa Magambo issued a Code Red when she feared the Stingrays would emerge from the wormhole created outside the Gladwell Road tunnel. (TV: Planet of the Dead)