They are lunar rainbows, in other words rainbows created by moonlight, not sunlight.
You don't see them in daylight, but in the night sky.
Just as a rainbow is sunlight refracted by raindrops, moonbows are moonlight refracted by raindrops.
You usually need a full moon, the brighter the better. The moon needs to be fairly low in a dark night sky, less than 42 degrees, and preferably lower (it also needs to be properly dark otherwise you can't see it, sunlight will obscure it). And of course you need rain. The raindrops then refract the moonlight, creating a moonbow, on the opposite side of the sky to the full moon.
Because moonlight isn't as brilliant as sunlight, moonbows tend to be weaker than rainbows. It's hard to see the different colours, moonbows tend to look white to the naked eye unless you are using a telescope.
