Because the mother of the groom isn’t part of the bridal party, this can be kinda tricky. You don’t want her wearing the same color scheme as your mother or bridesmaids.
Preferably, she should pick a color that she looks great in and won’t clash with the other women’s attire. Give her some suggestions, such as an accent color that is part of your overall wedding theme, and let her go from there.
Also, the mother of the groom’s dress doesn’t need to be as formal or lavish as the mother of the bride’s. In fact, let the bride’s mother choose her dress first so that the groom’s mother has a better idea of the style and formality of dress she should choose.
Don’t be afraid to give her your honest opinions and advice, after all, she’s asking because she doesn’t want to disappoint.