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Red was a colour at the opposite end to violet on the visible spectrum. It was situated after yellow and orange. (TV: Sky) Red was 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) Posicarians (TV: Mindwarp) and Eve's species had red skin. (TV: Sarah Jane's Alien Files episode 2)
Rarely, red was an eye colour. Ood would gain red eyes whilst being possessed (TV: The Impossible Planet) or whilst having lost connection to the hive mind (TV: Planet of the Ood), with the Shadow Architect and Servant of the Shadow Proclamation also processing red eyes. (TV: The Stolen Earth) Qataka's cybernetic body had red eyes. (PROSE: Timewyrm: Genesys)
Although at least some Daleks' eyepieces were incapable of seeing the colour, (PROSE: What To Do If A Dalek Attacks You!) several ranks of Daleks were identified by red casings, including the Red Daleks, (COMIC: Eve of War) a model of Supreme Dalek casing first used by the New Dalek Empire and later in Davros's rebuilt Dalek City, (TV: The Stolen Earth, The Magician's Apprentice) and the New Dalek Paradigm's Drone Daleks. (TV: Victory of the Daleks)
In Through the Eye of Eternity, the Red Eye of Eternity generally represented war or disaster, as portrayed through stock footage. After being served the wrong type of pizza, one fan drove his white Buick into a New York restaurant while screaming about the Eye being "on red"; in reality, the Eye pin he was wearing was green, but the man was colour blind. (PROSE: The Book of the War)
When Charlotte Pollard described Chief Stewart Weeks' nose as being "redder than Mr Lenin's pyjamas" on 4 October 1930, 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)
In the spring of 2007, while facing a Hoix with the Tenth Doctor, Rose Tyler threw a blue bucket of liquid at the Hoix. This was the wrong bucket, Rose had thought the Doctor said blue, but as the Doctor noted, he had said "not blue". This enraged the Hoix who chased Rose and the Doctor until Rose found the correct red bucket. (TV: Love & Monsters)