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. (DW: The Empty Child)

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