We know the bird is a male because he has reproduced with a hen. It would be good to know if any of the offspring end up appearing like the male, or perhaps breeding a female offspring back to the male?
There are also chimera birds... birds that can display more than one coloration due to cells from different zygotes.
https://en.wikipedia.org/wiki/Chimera_(genetics)
Every once in a while, birds may pop out with the colors of the opposite sex. The most "famous" ones are eclectus, although these birds typically appear half male, half female. These birds can be male/male, female/female, or male/female. Male/female birds might be infertile, unless one sexual organ is dominant, in which case the bird may be fertile. (at least, in theory)
I have heard of a normal grey male cockatiel, had all the right colorations for a male and all the behaviors. Well, since the bird was visually sexed (and seeing a photo, this bird did indeed look like a male cockatiel!), it was quite a surprise that "he" laid an egg.
A similar thing could be going on with this male plum head, or it could be a mutation, or maybe diet related. It's harder without knowing more about the bird's heritage or without doing any test breedings.