[...]
and Geralt with grey hair. FTFY
No seriously, if grey instead white hair is alright with Geralt and goes by the "close enough" rule, why would brown such a disaster for people? I mean we're talking about a game here that's it's own medium besides the books. Her hair is and always will be black in the books no one's going to take that away, so I honestly don't mind creative liberties taken by other mediums such as films and games as long as it's limited and doesn't deviate too much from the source.
I mean everyone who complains about one creative liberty is fine with another, that, in turn is dirt in the eye for some other guy.
However that's just my opinion and I can totally understand folks like you since in the books, her hair is not mentioned as a footnote like: "ah yeah, and she had black hair" but rather as a feature that defined her appereance.