Mauve alert

Mauve Alert is the universally recognised color for danger, signifying the highest level of danger, as acknowledged by most species in the universe. the Doctor and Rose picked up a mauve alert from a Chula ambulance that had been set to crash in the middle of World War II by Captain Jack Harkness. (DW: The Empty Child).

Red Alert
Red Alert signifies the highest danger for humans whereas the "rest of us", as the Doctor puts it, signify danger with mauve. (DW: The Empty Child)