I'd always assumed he was just a regular wolf but I was doing some research about having two different eye colors and found this interesting:

"In fiction, this often indicates a character with special powers, perhaps a shapeshifter or magical being of some sort."

^ Thats awesome! I thought he seemed magical.

There's something behind the fact that he has two different eye colors obviously...

"perhaps a shapeshifter"
That would be interesting especially since Graham seem to have a special connection with the wolf...