Good:
Really nice detail! I also love how the background and the black feathers are nicely blurred. You did a good job with all that!
The brightness and contrast that you adjusted seemed to work out just fine and there are no problems with that as far as I can see.
Not as good:
Lighting is a bit poor - the shadow across the head distracts.
Connection to the challenge - I actually see in the shot red, orange, yellow, (not green), blue, and purple. Some people were looking for rainbows that had the colors all in one place or row. I was just looking to see if all the colors were there, for the most part. So your image would have met the challenge to me, except that the image is dominated so much by the color red. I think that your biggest downfall as far as scores was the connection to the challenge.
For me this is a bit of stretch for this challenge. The photo seems a little static, compositionally - subjectively, I would prefer the turkey to be in partial profile, or if it is stare out of the shot, to have a little more space.
Interesting angle - hope he makes it through November without making it onto an American dinner table. :-) As for a rainbow of colors? Ehhh...bit of a stretch to me.
Never really thought of a turkey's head as a rainbow.. but I guess it sort of is! Good focus and angel. Would have been nice if you could avoid the shadow on his head.
Great depth of field. Not reall a great turkey shot though. The shade is distracting as is the angle of the head - would have been nice to see the eye more directly. Finally I can't see any rainbows on the turkey - really misses the subject for me.