Not all rainbows occur on rainy days. Rainbows can also form in the mist at the base of waterfalls, such as those at Yosemite Park in California. They occur not only in the daytime, but also at night, with a full moon at your back.
Nocturnal rainbows form in the same way as daytime rainbows: by means of light reflecting and refracting in droplets of water. At night they’re sometimes called “lunar rainbows” or “moonbows.” In this time lapse video, look for the reversed secondary rainbows at 2:40 and 3:00.
Direct link to “Yosemite Nature Notes’ video.
The Nature of Light and Colour in the Open Air (Dover Books on Earth Sciences)
Color and Light: A Guide for the Realist Painter