Just like its day-time equivalent, the lunar rainbow or ‘moonbow’ is created as light is refracted through water particles in the air, but instead of the light source being the sun it is now the moon.
Lunar rainbows are much fainter than their day-time forms with which we are all so familiar. This is because of the lower amount of light reflected from the surface of the moon. Only when the moon is full and skies clear of cloud is enough light reflected to create a moonbow, which always occur in the sky opposite from the moon in relation to the observer.
The human eye finds it difficult to discern the colours in a moonbow because the light is usually too faint from the moon to excite the human eye cone clour receptors (this is true of all night-time vision). Hence, moonbows can appear to some human eyes to be washed-out and white, without the concentrated colours of a day-time rainbow.
This varies according to the quality of the individuals night vision and the number and responsiveness of colour sensitive cones present in the retina of their eyes. So for some people it can be a bit disappointing, if your expectations of a bright rainbow are too high. Here is our advise; seeing a seventh wonder of the world, by magical moonlight is a surreal experience in itself. It is NOT just about the moonbow. The vibrant colours of a lunar rainbow do show very clearly in photographs with a long exposure. So for the avid photographer this is particularly exciting and challenging.