*in the books.
We are not in the books here.
And as CDPR already stated, Yens hair is black. If it is not black enough for your imagination, that's bad, but you cannot blame CDPR for not making her hair black. They have.
And for many people it is enough, I would say.
Yennefer has chestnut hair colour in that trailer, period, it´s not my opinion, it´s a picture, there it is what it is, I´m not picking on people who like that hair colour, but I´m saying if someone states, over and over again, their prime goal, it´s the fidelity to the world and characters, that´s a contorversy.
I remember when they said they don´t will put crossbows on the game, because it´s unfaithful to the lore, or horse combat, and there they are, and I can live with that, those are game features for those who want them, and doesn´t really "break" the lore, but changing a character, even only on how he or she looks, then I´m sorry but who agrees with that don´t know Geralt, Yen, Ciri and company near enough, their design, is so intimately related to who they are, and their charisma, that even a little flaw like that, from a studio that boast, again and again, love for the detail, from the tiniest leaf on the ground to the script, and fidelity, as fan, not only of the books, but the games and the studio for their previous work, and obviously the best if not the only good company policy in the industry of gaming...
Then as costumers and fans we have the right to speak our voices, always with respect, and them the right to answer if they see fit, and if they want to change something for whatever reason they only have to speak honestly, without PR tricks.