Mauve

Mauve was a colour that was nearly universally a sign of danger.

The mauve alert was a common distress signal. Humans used red alerts instead, which led to some confusion. (TV: The Empty Child)

The xXltttxtolxtol pods that were sent through the Cardiff Space-Time Rift were marked with mauve, implying they were toxic. In truth, the xXltttxtolxtol just didn't want Torchwood Three to open them. (PROSE: Risk Assessment)

When Charlotte Pollard described Chief Stewart Weeks' nose as being "redder than Mr Lenin's pyjamas", the Eighth Doctor disputed they were red; he claimed that they were a "sort of mauve" when the Doctor took a train with Lenin from Switzerland to Petrograd. (AUDIO: Storm Warning)