I find it tends to happen particularly if I've spent some time away from the fandom.

It's as though you go away, come back and suddenly somebody's post a truck load of rubbish all over your favourite fandom.
I've got a feeling though, that the truth is that Bleeprin has managed to erase most of the potentially insanity-inducing fanfic from the time before so I'm never quite ready for the shock when I return from my break.

it said that Hermione magically changed Rose's hair to blonde. Rose was part of some kind of "magical exchange program", or some such idiocy. Apparently, Mickey was a wizard as well.

Two traits involved: Brown-to-blond (the more brown, the darker the hair) and red/no red (no-red is dominant). (There are probably multiple genes involved, because you can get different shades of brown, from practically blond to deep black.)

A red-haired person (orangey red, not auburn) has to have a blond, red/red genotype, because red is a recessive gene and you can't have too strong of a "brown hair" trait before it looks auburn. They can pass on only blond and red to their kids. That means that if two red-headed people have kids, all the kids will be redheads.

Ron passes on blond and red/red. Hermione has brown hair, so she could pass on anything from blond to brown and either red/not-red or not-red/not-red.

So Ron and Hermione's child could have, depending on which genes Hermione is carrying, either red, auburn, blond, or brown hair.

The only hair color Rose Weasley couldn't have is deep black or very dark brown, because Ron and his red hair will only pass on "light hair" genes, which at the most end up as mid-brown if Hermione passes on all her dark-hair genes.

Oh, and this also means that anybody who writes a non-redheaded Weasley kid can be charged with forcing Molly Weasley to cheat on her husband, because that's the only way it could happen (barring adoption or transfiguration).