The colored wisp of cloud is too high in the sky to be a rainbow or a circumhorizon arc, and the spectrum is too ordered to be an iridescent cloud. I looked up ice halos that might produce this effect near vertical just before sunset. It turns out a circumzenithal arc is a perfect match: a rainbow arc near the zenith, brightest when the sun is very low. I’d never seen one before – it’s always cool to spot a new kind of sun halo.
Spotted November 12 at the Irvine Spectrum shopping center. It was around 90°F during the day at ground level, but of course it can be a lot colder in the upper atmosphere.
Saturation increased to show the colors more clearly.