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 a 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 the front and sit there. Also, coloured people were forbidden from staying in whites-only motels like the Sahara Springs Motel and were refused services at diners. (TV: Rosa)

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

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)

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."