Geralt's hair in game, and Ciri's as well, IS "white". I don't think it will be changed between now and launch, but as people mentioned, there will likely be someone out there who'll mod it to be whiter sooner or later.
Keep in mind the time period that the game takes place in. It is quite rare for a person to grow old. Between disease, starvation, war, monsters, nasty neighbors, bandits, etc it was pretty common to die before your hair had a chance to change color. So people seeing Geralt or Ciri with white hair would appear to be very strange to the average person. Even if it had a few blonde hairs or grey hairs amongst the white, they would likely describe it as white hair as it was so uncommon.
"Did you see that Witcher walk past?"
"A Witcher? Not sure, what did he look like?"
"Tough looking. Big scar on one eye. Long hair."
"What color hair?"
"Well.. sort of an off-bone color with gunmetal highlights. Possibly ivory with platinum strands woven in or even an ecru with a dusting of ash."
"Oh. Yeah, he went thataway."
And, just like in the real world, not the world of the Witcher, it is REALLY rare to meet someone with true white hair that is considered young. Some blonde hair is very blonde, almost to white and albino people due to lack of pigment have very white hair. Otherwise, it is almost unheard of (cue fans now lining up with pictures or stories of people they knew with white hair).