Here:
After being placed in stasis, the virus within Jill's body reactivated and soon disappeared completely due to the presence of powerful anti-bodies in her body, forged for years. Seeing this as an opportunity to make the deadly poisonous Uroboros virus less potent, Wesker continued to extract the anti-bodies from Jill's body for a period of time. Side effects of her experimentation included a pigmentation abnormality, leading to Jill's hair turning a light blonde, as well as skin etiolation giving her an extremely pale complexion.
http://projectumbrella.net/articles/Jill-Valentine
I can only wonder what the developers were thinking when they did this? Blonde hair and purple battlesuit just doesn't suit Jill. Were they deliberately trying to make her look like Nina Williams?
Was that in the Jill Valentine File in RE5?
Yeah, pretty much answered already, the testing done on her caused the pigmentations to go AWOL.
I definately preferred her as a brunnette.
That's the Jill Valentine article in Project Umbrella, i'm sure they have written it based on official texts, like the file you mentioned (and probably the Kaitaichincho guides, Archives I and II, etc.)
It is only to change her....
In fact I liked
Y'know, even though the answer to this question has already been given, every time I see someone ask why a character has different coloured hair, man.... sometimes I just want to quip something along the lines of 'because there's a substance sold globally that men and women use to change the colour of their hair and it's called hair dye'.
i like Jill as both brunette and blonde