Red

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)

The Tenth Doctor referred to Mars as the "red planet". (TV: The Waters of Mars)

On Gallifrey, pastures of red grass stretched far across the slopes of Mount Perdition. (TV: The End of Time)

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) The Shadow Architect and Servant of the Shadow Proclamation also had red eyes, (TV: The Stolen Earth) as did the Wrarth Warriors (TV: The Star Beast) and Qataka's cybernetic body. (PROSE: Timewyrm: Genesys) During his imprisonment beneath a pyramid, Sutekh was known to his worshippers as "the Red God". It was said that those who entered his services would gain red, bloodshot eyes. (PROSE: Background)

Although at least some Daleks' eyepieces were incapable of seeing the colour, (PROSE: What To Do If A Dalek Attacks You!) due to the fact that when the colour-corrected electronic eye of the Dalek was constructed, red was not included in its perception range, (COMIC: Strange to Tell... According to the Daleks) several ranks of Daleks were identified by red casings, including the Red Daleks (COMIC: Eve of War) and Supreme Controllers. The Dalek Survival Guide, noting the presence of red coloration within Dalek ranks, concluded that red was indeed visible to the Daleks, instead theorising that "the misunderstanding" came from the possibility that very early-model casings could only see in black and white. (PROSE: Dalek Survival Guide) A model of Supreme Dalek casing first used by the New Dalek Empire and later in Davros's rebuilt Dalek City used a red casing, (TV: The Stolen Earth, The Magician's Apprentice) as did 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)

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)

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. Ursula Blake also frequently wore red glasses. (TV: Love & Monsters)

In slang
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)

During the Cold War, the Americans and the British referred to their Russian and Chinese rivals as the "reds". (COMIC: The Good Soldier, TV: Dreamland, Children of Earth: Day One)

Welcoming Ashad and his Cybermen to the ruins of Gallifrey, promised that he would "roll out the red carpet", quipping that it was red because it was "drenched in the blood of [his] people". (TV: The Timeless Children)