The mother of the groom typically has to consider two elements of dress-buying etiquette: coordinating with the mother of the bride and choosing an attractive, appropriate dress. It's traditional to let the mother of the bride choose her dress first, then choose a dress that's similarly dressy and does not clash. As for choosing a dress that's appropriate, consult the bride before making a final choice. For a wedding with two grooms, both mothers should work together to choose dresses.
It's customary for the mother of the bride to choose her dress first, then share details with the mother of the groom. The goal is to pick a dress that looks "right" next to the bride's mother's dress. If one woman wears something short, light and flowery, for instance, the other may look out of place wearing something long, dark and heavy.
If the bride's mother does not choose her dress by four months before the event, or if the bride is motherless, the groom's mother is free to choose her own dress.
Navy is a common color choice for the mother of the groom. Avoid white, red and black. If possible, ask the bride what colors and styles she prefers to see the mothers wearing, and choose accordingly.
When both grooms are men, they should provide some dress guidance for both mothers, and both women should share photos of potential dresses before making final decisions.