Well done Momoftwoscallywags for being the first to confess to being 'that' guest in white!
I have to admit that despite what most PPs have said, I have genuinely never heard of not wearing white as a wedding guest until reading this post today. And I've done it three times that I can recall.
As a cousin of the bride, friend of the bride and SIL of the bride and had no idea I was doing anything wrong. At SIL's one, a relative did say to me "Why are you wearing white? You're not the bride", but I thought she was just being snide (other issues), not giving me etiquette advice.
In my defence, none of the dresses were floor-length, all button-up shirt dresses (80s/90s) and all accessorised with colours; green (double jeopardy!), pink, then yellow. And apart from jeans, I practically always wore white in those days so it hopefully didn't look like I was being a bitch, but just being me. I've also worn white to a funeral where the deceased had specified "no black".
I will now have to go back to these three brides and ask if it bothered them that I wore white, although I suspect they won't even remember now, especially as they are all divorced. Will apologise if required. 
An actual wedding dress is a different thing altogther OP. If she does go ahead with it then I do hope you can see the silly side of it in later years and not hold a grudge - life is too short.