I have never heard of being considered disrespectful or considered to hate the bride just for wearing a dress with the slightest bit of white either 😲! That's quite mad 😅.
Tbf, I also haven't seen that said by even the strictest people on Mumsnet...
I thought it was mainly just dresses with a white (or nearly white) background or, obviously, all white.
A navy dress with small white flowers, I would wear, for example. And, (for myself only - don't care what anyone else does), I'm fairly strict about not wearing white dresses as a wedding guest.
But, a white dress navy flowers, I would not wear. Or a white dress with big pink flowers. You get the idea!
My rule for myself is, if someone asked me what what I was planning to wear and if I was describing my dress, if I naturally said "it's a white/cream/off-white dress with....", or "it's a white background with...", for me, that would be one to avoid. But if I said, "it's a blue/pink/orange/purple/whatever colour dress with some white flowers/lace/spots/details", then it would be fine (for me)