Not a rainbow, this photo is of low clouds that carried moisture. As they evaporated over the eastern ramparts of the Rocky Mountains, the late afternoon soon created this “cloud bow” effect.
Last weekend I was at a training session in Boulder, Colorado given by a publisher I work with. The training was very exciting, but I didn’t get much chance to capture photos. Monday afternoon after everyone left I went for a short hike on Flatiron Mountain above Boulder. And found this phenomenon.