This is not a rainbow. It's a moonbow, an extremely rare atmospheric phenomenon caused by the near-full moon that is extremely hard to catch. So hard, in fact, that you can only see its colours thanks to long-exposure photography.
It was captured by Wally Pacholka last January 20, at the Haleakala Crater on the Island of Maui, Hawaii. The moonbow - or lunar rainbow - is caused when the near-full moon at less than 42 degrees in a dark sky. The colours are so faint that the human eye colour receptors can't be excited enough for the brain to identify them. Therefore, they appear as white arcs to the naked eye. Only by using long-exposure photography you can reveal the diffraction of the moonlight through the microscopic water droplets suspended in the air.
By the way, that red thing shining on the sky? It's Mars rising. Check out the rest of Wally's amazing images at [Astropics]