The MOB looks great in her beige, but you look awesome in that black number. Good for you. Same thing happened to me last summer. GS was getting married and I was told the grandmas/grandpas were walking up the aisle as part of the procession. Well, I have white hair and don't like the way I look in beige or brown (I do wear it, but I look better in other colors.) So I shopped and shopped and came up with zilch. I decided to get black with a white linen jacket. As you said, they were surprised, but the "show" went on and I looked nice.
BTW -- cute couples