Sky illuminated with colours in Vail, CO, Friday, Aug 17, 2001.On Friday, 17 August 2001 some time around 20:30, a friend and I were in Vail, Colorado looking southward toward the planet Mars. Our view of the horizon was obstructed by mountains. The view to the North was obstructed by a building. For a period of 4 to 5 seconds, the entire section of the sky that was visible to us was lighted by a bright light whose source I could not see. The light changed colours, including blue, white, yellow and orange, though I don't recall the sequence. It appeared as if the source of the light was due South, below the mountain ridge, but since my view North was obstructed it is possible it originated in the North. We watched the sky for another 10 minutes but saw no further occurrence of the light.