The brief for the art war did state that you need to: "create the most epic champion the art world has ever seen"
Many people can obviously take this different ways, but for me the character would need to ooze power, or at the very least confidence and ability. I agree that your mouse needs personality, but I would push it more towards a warrior cast, at the moment he is reading more "travelling bard" to me.
To summarise I think he needs to radiate confidence and bravery (You don't need to deck him out with muscles or anything, that's not what I mean.) Perhaps look at the cover illustrations for the Redwall series of books. Whoever does them has a talent for making mice characters look imposing.
As for colour just look at real mice and pick a scheme you like. Maybe albino?