It's an unspoken rule for wedding guests to not wear white. White is reserved for the bride. It's her day, don't try to upstage her.
White stands out. That's why the pope wears white, and no one else. Think of the bride at a wedding.
So when you belong to a religious order, whose habit is white, you wear your black cloak over it. That's why Dominicans wear their black cappa over their white habit when they are in the presence of the pope. Now you know.
