As the mother of the bride, you will be giving a nod to wedding etiquette when you select your dress for your daughter's wedding. While you do have some leeway, you will need to pay attention to your daughter's chosen colour theme and the formality of the event.
Bride's Mother First
Anne Gamel, Weddings 911 columnist, says that the mother of the bride chooses her dress before the mother of the groom. Gamel says the reasoning behind this tradition is that only the bride should outshine her mother. The mother of the groom is not allowed to overshadow the mother of the bride. Both mothers (bride's and groom's) should select dresses that complement the bridal party as well as each other. Keeping this in mind, the mother of the groom should not select a short dress when the mother of the bride has selected a floor-length gown.
The bride will let all bridal party members know what her colour scheme is. If she selects burgundy and white, her mother would buy a dress in light green-yellow (chartreuse). In order for the mother of the groom to follow wedding protocol, she would have to select a formal dress in a colour that complements the burgundy, white and chartreuse colour theme. The reason the mothers of the bride and groom should coordinate their dresses to the bridal party is so they don't "clash" when wedding photos are taken. As the mothers are considering which dresses to buy for the wedding, they need to make sure they complement each other while they are paying attention to the dress styles being selected by the bride for her dress and for her wedding party. Their dresses should feature one predominant colour and be of a similar length.
In general, at a formal wedding, the mother of the bride/groom dresses should be formal and floor length. At the very least, they should both choose tea-length dresses. If the bride and groom have decided to go with a more semiformal or informal wedding, the mothers can be a little more creative--but they still need to make sure their dresses coordinate with each other and with the bridal party.