Fenrir is, to my knowledge portrayed only as a black wolf in Norse Mythology. 

You [i]can[/i] find pictures of Fenrir appearing as a white wolf, but that's in Final Fantasy. Furthermore, Lily's full shift is a large white Fox with nine tails (obviously), and I think having a bit of diversity would be a good thing. While they are fundamentally different, I still feel like they're a bit too similar.

How about a compromise instead? [url=http://fc04.deviantart.net/fs71/f/2012/115/d/7/fenrir_bound_by_poledrey-d4xiixa.jpg]http://fc04.deviantart.net/fs71/f/2012/115/d/7/fenrir_bound_by_poledrey-d4xiixa.jpg[/url] [url=http://img3.wikia.nocookie.net/__cb20130127003258/finalfantasy/images/b/b5/Fenrir-FFVIIAC.jpg]http://img3.wikia.nocookie.net/__cb20130127003258/finalfantasy/images/b/b5/Fenrir-FFVIIAC.jpg[/url]

I found these two pictures, both portraying him as having an icy grey fur. It isn't quite white, but it isn't black either. He /is/ a descendent of Frost Giants (Loki, his father, was the son of the Giants Fárbauti and Laufey) so an icy colour fits fairly well, I think. It's different enough that I'm okay with it, but still somewhat white/light in the colours like you want.