DAT SIG.Hmmm...maybe it's intentional? maybe Ghadius is fading away, over the course of the game, as indicated by the change in eye color, or he's givin all his strength into awakening Nahatomb? Either that, or they couldn't decide on a specific eye color, and ended up getting the txtures mixed, or something.
Though, I'm sticking by my "changes over the course of the game" theory, if only because...
Shadow of The Colossus Spoilers:Spoiler
Sorry but you are not allowed to view spoiler contents.