A regular rainbow has the red on the "outside" of the bow and violet on the inside.
Occasionally, one may see a secondary, dimmer, "reverse" rainbow where the color order is reversed where red is on the inside of the bow and violet is on the outside.
In most cases, one would see both. But, what if one only sees the secondary one? Can he make a bracha on this? Or would we assume that the bracha on the rainbow means, by definition, a rainbow that has its colors arranged in the "normal" order?
See this for a picture and explanation of this phenomenon.
Note: We have a few "rainbow" questions on this site. In particular, this one discusses rainbows created by cirrus clouds - probably more common than the double rainbow mentioned in this question. It's clear (sorry for the bad pun) that in my question, the rainbows are caused by rain-producing clouds. I have never seen double cirrus "rainbows", and given the "reverse" color arrangement, I don't think ice crystals would produce that type of refraction.