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This is a complicated question with an answer that depends on the time of day, the formality, what the bride's mother will wear, and where the wedding will take place. For example, in New York, no one would think twice about the groom's mother wearing black, especially if the wedding were after five. But I can't imagine it being okay amongst my parents' friends in the south. If the wedding is before five, or isn't formal, I'd say forget it. Otherwise, I'd start out by talking to the bride's mother about that she is wearing and see how she feels about you wearing black. You might also consider soft gray, or a jewel-toned purple or green, which will look wonderful with the bridesmaids' claret dresses.