Rainbows are a bright spectrum of light that appears in the sky when light reflects off moisture in the air. Double rainbows are less frequent, less bright and usually sit outside the first rainbow. They occur because of a double reflection of sunlight off moisture and appear at an angle of only 50 to 53 degrees.
When refraction occurs to cause single rainbows, the light strikes and goes through moisture. Double rainbows are created in the same way except the sunlight bounces twice off the moisture before re-entering the sky. The second rainbows are fainter than single rainbows. This is because the light has been reflected twice and had two chances to be transmitted out the back of moisture in the air.
This second reflection inverts the colors when dispersion occurs. The region between a double rainbow is dark.