You should be able to change the background color of all bags, but I don't know if you'll be able to have different colors for different bags. Each bag is a clone (a copy) of the same basic window XML. You might be able to do it by putting a color selection feature into the bag window (so that it then changes it for just that bag in-game) but I'm not sure if it would save it between sessions. One of the folks that is better versed in UI modding might be able to shed more light there.
__________________
|