I'm not sure I'd consider the Archbishop a force of Light; he may *think* he is, but he and his knights listened to the Ascians and became Primals...something that doesn't really follow the Light (by a similar stance, Iceheart can't be fully considered a force of Light either, despite actually having the Echo).

Just because you think you are something, doesn't mean you are.

I can't say for sure he was Dark either; he was against the Ascians and at least tried to kill Lahabrea (I don't think he's really dead just yet). Misguided, yes; he wanted to protect his people though in a twisted way.