I would ask the bride what type of attire she wants you to wear. I would imagine that a dress would probably be what she has in mind. Typically the mother of the groom is pretty dressed up since she is a part of the wedding. Perhaps you could wear a tea length dress in a color that will work well with the wedding colors. Traditionally the mother of the bride shouldnt wear black, any shade of white/off-white, or the color of the bridesmaids dresses. However, some brides dont mind if the mother of the groom bends those traditions a bitjust check with the bride to see what she has in mind for you to wear. Enjoy the wedding!