Been thinking further.......
I remember a wedding a few years back where the MOB was FUMING when the MOG let her know that she was planning on wearing (say) blue. She was a fair bit older than the grooms mother and was very vocal on how MOB gets the first choice on colour for outfits, about how even the Queen waited until Carole Middleton had chosen her colour for the wedding before deciding herself.
I think these days, given that there are few fewer weddings than there used to be, and also there are so many weddings where guests are specifically asked to wear a certain colour, that so called "rules" like this have been forgotten. The only place I have come across them in recent years (50 this month) is on MN and wedding websites (DD is helping her best friend organise her wedding.....wow thats been a lesson in self obsession, some of those brides are INSANE!)
I think its important to remember that these days a lot of so called "rules" are ignored in favour of comfort. My own mother (in her 70's) wore jeans and a jumper to the funeral of one of our family friends last year. She would never have done that a few years ago. She said "Oh who cares? Jane wouldnt mind and anyone else who does can bugger off" and no, she doesnt have dementia before anyone asks she has just learned, as we all have in recent years, what matters and what doesnt.