Moonlight refracted in a passing cloud at about half-past midnight. The moon is too overexposed to be distinguished, and the cloud is blurred by its own motion during the moderately long exposure required to capture its subtle colors, but the rainbow of colors in the cloud is clearly visible. (A little saturation enhancement helped, too.)
I can’t recall having seen this phenomenon before. Maybe I just haven’t paid much attention to such things heretofore, or maybe there’s something a little unusual about the cloud. It’s cold here tonight, so perhaps the cloud is composed of ice crystals, each functioning as a tiny prism. Pure speculation on my part—this isn’t one of my fields—but, whatever the cause, it seemed to be worth trying to get a photo before the cloud passed.