Phoenix actually changes colour after the add phase in the battle during the Final Coil of Bahamut. Once Phoenix finishes casting the "Rebirth" skill and enters the final phases of the fight, its blue feathers turn the red-orange. It's the latter colour in Unending Coil as well, but I'm not sure you can confidently point that "blue feather = tempered" when Louiphoenix still pounds away with us at full force in the actual canonical encounter of Final Coil as the red-gold Phoenix.