Rainbow. I think this phenomenon is called sky fire or fire rainbows? I saw one in the sky over Chicago (April 2013). It lasted about 40 minutes and was truly awe inspiring. It changed color and intensity slowly and at times was only partially visible behind a veil of bright cumulus clouds. I was driving to work at the time so was unable to take pictures.
Photographer captures rare 'fire rainbow cloud' above Florida as Mother Nature puts on spectacular light show. Ken Rotberg spotted unusual phenomenon as sun dropped behind a storm cloud in the early evening.