I've never heard of the Mother of the Groom having to tone her outfit to the bridal party. I know, when my brother was getting married, my mother checked out if the bride's mother was wearing a hat as, if not, she didn't want to show up in one and look a bit overdressed. But they wore completely different colours and styles of outfits. Apart from the main group photo, the mother of the groom isn't usually photographed with the bridesmaids anyway (unless one of them is her daughter).