I have written before on:
Here is another example I came across recently: a mutant form of the green-cheeked conure (Pyrrhura molinae):
Here is a bird of this species with normal plumage:
And here is another mutant green-cheeked conure (the “suncheek” mutation on the left, with a normal bird on the right); note the juxtaposition of orange and yellow on the plumage of the mutant bird.
More information on this mutant here.