I have the opposite problem to Jolene. My dress is full length with train, I would like my bridesmaids in tea length, but my mom wants to wear full length. Haven't yet consulted MOG. If my maids are in tea length, can the MOG/MOB still go full length?
Typically, the bridesmaids would be dressed as formaly as you, which would mean long dresses. Yours is formal. But, you are the boss here. So, if the bridesmaids are wearing tea length, the mothers should be too. But again, you are the boss here. Rebecca Black, Etiquette Now
Etiquette says that the MOB/MOG dresses should compliment the bridesmaids color and length. If they want to wear formal length gowns, which is most popular, that is fine. It would actually match the formality of you dress.
Since the length isn't going to match the bridesmaids dress length, the color the MOB/MOG wear should. MOB/MOG shouldn't wear the same color as the bridesmaids, just a color that compliments them.