Can't find it, sorry about that, but I was fascinated by it. I have just played bridge with a load of feisty ladies in their 70s and I asked them how they would feel. They were appalled and said they would either not go to the wedding or would wear what they wanted which would NOT be black and white. As women get older and more faded, they need colour (so I am told) and colour becomes more and more important.
I have asked your question to women of your mil's demographic and they were uniformly and heatedly appalled at the suggestion.
They all, to a woman, think you are being very unreasonable.
I think it's weird but I may well be v out of touch.