A moonbow (also known as a lunar rainbow or white rainbow), is a rainbow produced by moonlight rather than sunlight.Moonbows are much fainter than solar rainbows, due to the smaller amount of light reflected from the surface of the moon.
Because the light is usually too faint to excite the cone color receptors in human eyes, it is difficult for the human eye to discern colors in a moonbow. As a result, a moonbow often appears to be white. However, the colors in a moonbow do appear in long exposure photographs. (Source: Wikipedia: Moonbow)