The classic example is a peacock's tail. But sexual selection could also serve to separate species in what is more-or-less a one-step process, by the sudden appearance of such a characteristic.
Ellen had a beautiful peacock-feather fly-brusher, but it was used only on very special occasions and then only after domestic struggle, due to the obstinate conviction of Pork, Cookie and Mammy that peacock feathers were bad luck.
The green curtains with their peacock design, hung in straight lines, and the green carpet, in the pattern of which pale rabbits frolicked among leafy trees, suggested the influence of William Morris.
So peacocks get all the attention when it comes to iridescent plumage, but I think that the fanciest rainbow feathers belong to the smallest members of the living dinosaur family, hummingbirds.