Colour

The Silhouettes were allergic to colour. (COMIC: Sunscreen)

In the America, from at least 1943 to 1956, people who were classed as non-whites or of colour were segregated from several places in states like Alabama. For instance, coloured people were segregated on the bus services, forced to sit at the back and get on through a side entrance while whites could get on at the front of the bus and sit there. Also, coloured people were forbidden from staying in whites-only motels like the Sahara Springs Motel and were refused services at certain diners. (TV: Rosa)

Mrs Smith's boarding house had a "no coloureds" sign, much to Ace's disgust. (TV: Remembrance of the Daleks)

Addressing humankind shortly before the Battle of Canary Wharf, Cyber-Leader One announced that their upgrade to cyber-form would remove, among other things, colour. (TV: Doomsday)

The Doctor's first two incarnations were colour blind. The War Doctor noticed the new colour balance in his eyes after his regeneration. (PROSE: The Day of the Doctor)

Immediately after regenerating, the Twelfth Doctor complained that he didn't like the colour of his new kidneys. (TV: The Time of the Doctor) When he later returned to Gallifrey and deposed Rassilon, the Doctor asked the General what colour the Hybrid, who was thought to soon bring about the fall of Gallifrey, was. The Doctor took the General's inability to answer this as indicative of prophecies being useless. (TV: Hell Bent)

When about to be fired down to the Dalek Asylum, Rory Williams questioned what colour the Dalek variants held within were. He defended this by claiming that there "weren't any good questions left". (TV: Asylum of the Daleks)

Behind the scenes
In DWM 477, showrunner Steven Moffat jokingly answered one fan's question on what colour the Doctor's kidneys now were, as "Froon. This is an entirely new colour, which only the Doctor can see."