Ruby was a shade of red named after a certain gemstone.
The Fourth Doctor wore a ruby cravat when he met Magnus Greel in London during 1892. (TV: The Talons of Weng-Chiang)
The Seventh Doctor wore a ruby tie with his suit of cream linen during an adventure in the System. (PROSE: Sky Pirates!)
Nyssa wore ruby shoes during her second encounter with the Mara. (TV: Snakedance)