At the risk of becoming the all-rainbows-all-the-time blog, I had brave monotony to share one more. Though these rare formations have the familiar red-to-violet spectrum, they are technically not rainbows, but “circumzenithal arcs.” They owe their upside-down shape to light refracted by ice crystals high up in the atmosphere. Note also that the colors appear in reverse order, with violet on top and red at the bottom.
Circumzenithal arcs are about as common as rainbows, but they seem rarer; because they appear at such high altitudes, they are harder to spot. A good place to look for them is in cirrus clouds.
One more reason to look up more often: you might see the sky smiling back at you.