Mauve alert

Mauve Alert was the universally recognised colour for danger, signifying the highest level of danger, as acknowledged by most species in the universe.

History
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. Rose believed red alert to signify the highest danger, but the Doctor informed her that was only for humans, whereas the "rest of us", as he put it, signified danger with mauve. (DW: The Empty Child)