Rainbow cloud in Denver
© Mike Quaintance
There were several reports of 'rainbows' and 'fire rainbows' in the sky on Friday but these terms are misleading because they were not rainbows, nor associated with fire.

To get a rainbow you must have rain clouds with the sun at your back.

What was happening is something we call iridescence caused by iridescent clouds.

An iridescent cloud is any cloud that exhibits brilliant bright spots, bands, or borders of colors, usually red and green, observed up to 30 degrees from the sun.

The coloration is due to the diffraction of light with small cloud particles which produce the effect. Iridescence is usually seen in thin cirrostratus, cirrocumulus, and altocumulus clouds.
Rainbow cloud Denver
© Steve Kady
Most of the clouds over Denver Friday afternoon were high cirrus clouds that likely had more ice than water particles inside, which helped scattered the light much like a prism would, creating the cool effect.

A few people also saw a sun dog which is a halo around the sun with a distinct bright spot on either side. These are often seen when the sun is low on the horizon.

Sun dog Centennial, CO
© Noah Brauer
Flat ice crystals inside the cirrus clouds help produce this phenomenon.

Denver sun dog
© Ashley Michels
Rainbow cloud Denver
© MAD Hippies Life