Colorado was a state of the United States of America.
The Eleventh Doctor, Amy, and her fiancé Rory visited it in 1981. They spent most of their time in Appletown. (PROSE: Nuclear Time)
Tulung's father was from Colorado, before he moved to Alaska. (AUDIO: The Land of the Dead)
Harker's Pond was a town in Colorado, near Garden of the Gods national park. In 2011, Rex Matheson visited Harker's Pond in his search for Cotter Gleason. (PROSE: The Men Who Sold the World)