Rainbows occur when sunlight is refracted as it enters, totally internally reflects and finally leaves water drops suspended in clouds.
The water drops have to larger than normal cloud droplets. Light from the background comes straight to you with out being scattered as it would be for fog or a cloud.
There are different colours because the red light travels faster through water, is refracted less and is at the top of the rainbow. Blue slows more and is bent further.
Because rainbows are reflected light you can enhance them with polaroid sunnies or completely turn them off by turning your sunnies through 90 degrees.
As you can see in the first photo there is a second more dispursed rainbow as the light takes a trip throught the rainbow.
You can also see repeated rainbows ans general cloudiness beneath the rainbow.
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