Red was a colour used by humans, in the form of red alerts, to indicate danger. The Ninth Doctor said non-humans regarded it as "camp". (TV: The Empty Child)
Racnoss, (TV: The Runaway Bride) Blowfish, (TV: Kiss Kiss, Bang Bang) Zocci (TV: Voyage of the Damned) and Posicarians had red skin. (TV: Mindwarp)
Red Daleks (COMIC: Eve of War) and the New Dalek Paradigm's Drone Daleks (TV: Victory of the Daleks) were identified by red casings.
In 1893, Madame Vastra and her partners, Jenny and Strax, investigated "the Crimson Horror" — a mysterious condition leaving victims with red skin and preserved like statues. (TV: The Crimson Horror)
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)