Moon-bows form in much the same way rainbows do, but as their name suggests, they emerge at night when moonlight shines on water droplets. But that’s just the start conditions need to be just right before you’ll see one appear in the night sky.
“Moon-bows are rare because moonlight is not very bright, A bright moon near to full is needed, it must be raining opposite the moon, the sky must be dark and the moon must be less than 42d high. Put all these together and you do not get to see a moon-bow very often! To the unaided eye they usually appear, as in the small image, without colour because their light is not bright enough to activate the cone colour receptors in our eyes. Nonetheless, colours have been reported and might be seen when the moon is bright.
It’s no surprise, then, that moon-bows tend to appear more frequently in certain spots around the world where these conditions are more likely to line up. Because they’re so faint, they often appear white to the human eye but the colours come to life in long-exposure photographs.