More responses....
Daema I was also going to say possibly liver damage. I'd run some blood panels just to be safe. Liver damage can cause discolored feathers, such as red or yellow coloration where it shouldn't be.
Also, Pionus and Amazons are two entirely different genus, they can't hybridize. Just like how a Macaw and Cockatoo can never hybridize naturally.
Myself Guess a good question may be...
How do we tell the difference between "chocolate raspberry" (disregarding species) and red suffusion on an unknown bird?
The red suffusion I've seen doesn't come with blue hues. This amazon is *very* blue! That's what's confusing me!
Agreed though on species family. I *want* to lean towards other species, but that beak... just wrong shape/size and color for those other species.
Tony The plumage structure is key in identifying these clinical birds. The feathers have a curled like shape for one.
Tony has pictures of a few amazons with similar coloration in his FB album....
Mutation and Rare Parrots from Around the World | Facebook
So if we could get more photos of Tango, pretty please???
If Tango's coloring is in fact caused by liver failure, and the liver is healthy(er), then Tango should, in theory, return to normal coloration.
Besides the people who say pionus, we agree with your vet in that Tango belongs to the
Amazona ochrocephala complex. (yellow headed amazon of some sort)