Thank you to Wrench for his explanation...I had never heard of what the OP's Vet had described, and after reading their explanation I did my own research on this, and Wrench is exactly correct, and what the OP was told by his vet is completely false. I think it's important to correct the info given in the post by the OP's vet, who I'm hoping is not an actual CAV or Avian Specialist, because if they are, then they simply just made that entire explanation up and told it to the OP, which is not good and I'd definitely find a new CAV immediately...
***Contrary to what the OP's Vet told him, no Amazons have the ability to purposely "change their feather colors due to stress", nor does any other bird that I'm aware of.
The reason this Amazon's feathers turned brown was simply because they were wet at the time, and had not completely dried yet. So to anyone reading this thread, the information given to the OP by his Vet about "some Amazon's having the ability to turn their feathers brown when they are under stress" is completely false...
I'm sorry that your vet told you that, they either just made something up because they didn't know what was going on, or they just didn't have a clue what was going on...Either way, you definitely need a new vet, and I wouldn't ever take your bird back there...