i read about the tres maries a few times. the best website i found even gave good pics of both the tres marie and the DYH. the biggest difference is that the DYHs have alot of the black barrring on their chests and bellies, and the tres maries dont realy have much, if any, barring, and they have a noticeable blue-turquoise tint to their belly feathers. i would have to look it up and see if i can find it again...lemme go look...
here is one site, but there was another one that was really good also:
Tres Marias Amazon species profile
here is another one:
Yellow-headed Amazon - Wikipedia, the free encyclopedia