I'm a little confused as to your wording of this question. An oxidizing agent, by definition, is a molecule that readily transfers oxygen atoms. So exposing it to oxygen wouldn't cause any color changes, because oxygen is already present.
Vicinal dibromides, when dehalogenated, give off Br2, which is a brown liquid at room temperature. Dehalogenation is a reduction reaction though.
I'm probably missing something obvious here. As it is though, I have no clue.