Wedding usherettes' dress...

My close friend is getting married soon and she wanted me to be a part of her wedding entourage as one of the usherettes. I know it's not really a big part though but I just wanted to make sure I would not make myself a clown and somehow spoil the whole picture.. lol well, am wondering if it's just fine to dress in a while slacks and smart purple/lavender, or I really need to dress up and come in a formal lavender dress? please advise... their motif is lavender, by the way...:P Thanks...:P