Of course it's not only a Mumsnet thing, though I suspect it may be an English (or perhaps British) thing.
It's certainly been a 'rule' ever since I started going to weddings and I'm in my 60s. Similarly, I would consider black to be inappropriate.
The exception is for bridesmaids, if that is what the bride chooses. Going by the old photos I have of family weddings (going back to the 1920s) I'd say white is probably the traditional colour for bridesmaids.
In the US it doesn't seem to be the case. We have been to a couple of weddings where a number of guests wore white or black. I wonder whether it's because weddings there may be seen more as evening events and people dress accordingly: men in dinner jackets and women in cocktail-type dresses.
Evening dos here (England) seem to be increasingly important these days. In the 80s lots of the weddings I went to didn't have evening dos at all. I don't think I've been to a wedding in 30 years that has ended with the traditional send-off to the bride and groom leaving in their "going away" outfits.