My fiance's mom asked me what she should wear. What do I tell her?
Because the mother of the groom isn't part of the bridal party, she should avoid choosing a dress in the same color as the bridesmaids' attire, the mother of the bride's dress, or the wedding gown. Instead, she should pick a shade that both flatters her and won't clash with the other women's clothing in the photographs.
Typically, the mother of the bride selects her outfit first, then calls the mother of the groom to advise her on color choices. You might also have suggestions -- tell the mothers about any accent hues in your color scheme, or provide a fabric swatch of the bridesmaids' dresses for comparison.
According to Mara Urshel of Kleinfeld Bridal, a dress shop in New York City, pale pinks, greens, and blues are all popular colors; black is appropriate for only the fanciest weddings. The hour and formality of the wedding (not necessarily the style of the bride's gown) should influence your mother-in-law-to-be's final choice.