Sky

A sky was the visible part of a planet's atmosphere.

The sky of Gallifrey was burnt orange at night. (TV: "A Desperate Venture")

The sky of an Earth-like planet was green, (PROSE: The Sour Note) as was the sky of Arcopolis. (PROSE: The Eyeless)

The sky of an Earth-like planet was purple. (PROSE: Follow the Phantoms)