The mother of the groom should ask the bride's mother what color she will be wearing. Or ask the bride what colors will be used for her bridemaids. There are only two colors she shouldn't wear - white or black. White, because only the bride should wear white. Black because it means mourning and women shouldn't be wearing black for a happy ocaision. Most mother wear blue.
Although in these modern times of looser protocols, the mother of the bride and groom can choose to wear black if the color theme of the wedding is black/white. Ivory, cream, off white could then be considered the "white" colors to wear without necessarily wearing white.