My future mother-in-law asked me 3 times if it would be OK to wear black to my wedding. I just repeated, "Wear what you're most comfortable in." "Wear what you like." "I'm sure whatever you select will be great." ["comfortable," even for my formal wedding, was OK in the context, which included discussion of whether guests should buy new clothes. I don't think it implied "casual."]
So imagine my surprise when my fiance told me his mother had bought a white dress for the wedding. I laughed at the irony that the "no black to weddings" rule, now mostly ignored, had somehow gotten through, but not the still-in-effect "only the bride wears white" rule.
Further context: My future in-laws are immigrants who never fully assimilated into American culture. They're from the USSR, which of course suppressed a lot of wedding customs the family might otherwise have. So they're (mostly very kindly) feeling out this whole wedding thing. It's fascinating what things they pick up, like no-black, and some idea I should have a diamond in my wedding band because there isn't a diamond in my engagement ring, and what they don't.
My fiance told his mother to buy a different dress before he told my about the white one. His problem with the white dress was something other than it being white, so, we'll see. I just never in a million years would have guessed I had one of THOSE mothers-in-law who wears white. I've never seen her in white otherwise, not even in pictures, not even for her own wedding.