Red alert

Red alert was used by humans to signify the highest level of danger.

When the Ninth Doctor and Rose Tyler 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 commented on there being a "mauve" alert. She 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. He also stated that a lot of aliens saw red as camp — "all those red alerts, all that dancing." (TV: The Empty Child)

Despite this, the Titanic, from the planet Sto, used a red alert to signify asteroid impacts were imminent when the shields were down. (TV: Voyage of the Damned)

A flashing red light also meant danger on the Eleventh Doctor's TARDIS. When Clara Oswald encountered one such alert, she opened the door anyways and witnessed a huge fireball filling the corridor. (TV: Journey to the Centre of the TARDIS)

In a parallel universe, red alert was used by the Starfleet of the United Federation of Planets, of which Earth was a member. On Delta IV, a red alert was called within the secure command center when the planet was attacked by the Borg-Cyberman Fleet. Aboard the USS Enterprise-D, a red alert was called when the ship discovered the fleet in orbit of Delta IV. (COMIC: Assimilation²)