At 3:15 PM on a clear sky, condensation trails formed behind my plane at 20,000 to 30,000 feet. At about 3,000 feet, there was a cloud about 1/2 mile long and nearly 500 ft wide. The thing about this cloud was when I first saw it, it was very thin and white. It was like a condensation trail, but too low and moving too quickly. Get this--it started changing colors, every color you can think of for the length of the cloud (lasting about three minutes) and then back to white! Do you have an answer for this one MY FAVORITE WEATHER MAN?
Your favorite weather man is not available, so you'll have to settle for me this afternoon.
What it sounds like you observed (based on your note below) was a high-level stratus (cirrus) cloud, composed essentially of all-ice (rather than water droplets) . . . and the phenomenon you witnessed is sometimes called a "fire rainbow."
The process is similar to the "prism" (bending) light effect seen in a rainbow, except the ice crystals in the cirrus cloud are doing the work instead of raindrops. Like rainbows, you only see these when the sun/cloud/eyeball angle are all just right. So, as the cloud moves, if you don't move with it, the effect fades in ... then fades out.
The ice crystals of the cloud selectively bend the sunlight striking them, giving the "colors" appearance. The two examples (URLs) listed above - especially the 2nd one - are topshelf, "Grade A" examples of this phenomenon. More often, the rainbow effect is not quite as brilliant.
Of course, I didn't see it myself so I can't be 100% sure of my explanation ... let me know what you think!