Copper was a colour.
Gallifrey had at least two large moons, one of which was the copper-coloured Pazithi Gallifreya. (PROSE: Cat's Cradle: Time's Crucible) Quinnis had a copper-coloured sky. (AUDIO: Quinnis)
The Lakhotha (PROSE: Scribbles in Chalk) and the Rhumons had copper-coloured skin. (PROSE: Twilight of the Gods) The Jaftee had copper-coloured hair. (PROSE: Shining Darkness)
The steward of Platform One wore a copper-coloured robe. (TV: The End of the World)