Green

Green was the official colour of mourning in Perfugium, as it symbolised death and a return to nature. (AUDIO: Master) and the universal colour for medicine. (COMIC: A Confusion of Angels)

It sat between blue and yellow on the visible spectrum. (TV: Sky)

Time Lords of the Arcalian Chapter wore green ceremonial robes. (TV: The Deadly Assassin)

Sontarans had green blood. (TV: The Two Doctors)

Many species had green skin, including Alpha Centaurans, (TV: The Curse of Peladon) Etydions, Foamasi, Ice Warriors, (TV: The Curse of Peladon) Jadeans, Jagaroth, (TV: City of Death) Kleptons, Primitives, Rutans, (GAME: The Gunpowder Plot) Silurians, (TV: The Hungry Earth) Swampies, (TV: The Power of Kroll) Urbankans, (TV: Four to Doomsday) Vinvocci (TV: The End of Time) and Watermen. (COMIC: Dr. Who Meets the Watermen) Infant Chimerons (TV: Delta and the Bannermen) and larval Wirrn (TV: The Ark in Space) were also green.

Nardole noted that, while he had been blue at one point, he had never been green. (COMIC: A Confusion of Angels)

Stahlman's ooze was green. (TV: Inferno) The waste from Global Chemicals' oil-refining process was green. (TV: The Green Death) The sky of an Earth-like planet was green, (PROSE: The Sour Note) as was the sky of Arcopolis. (PROSE: The Eyeless)

Green was also an eye colour. River Song had green eyes, (TV: Let's Kill Hitler) as did as the Eleventh Doctor. (TV: The End of Time) The Second Doctor had green eyes (PROSE: Timewyrm: Apocalypse) which sometimes changed to blue. (PROSE: Invasion of the Cat-People) The Grand Matriarch had green eyes. (PROSE: Timewyrm: Apocalypse)

The Eighth Doctor remembered watching a meteor storm in the sky above him during his original incarnation. He said that "[t]he sky [...] was dancing with lights; purple, green [and] brilliant yellow." (TV: Doctor Who)

Green Daleks were at the Gallifreyan warship Starbane. (GAME: The Doctor and the Dalek)

In Through the Eye of Eternity, the Green Eye of Eternity generally symbolised that the week's episode would concern ecological themes. (PROSE: The Book of the War)

Behind the scenes
In the comic adaptation of the film Dr. Who and the Daleks, the police box-shaped TARDIS is erroneously coloured green as opposed to TARDIS blue.