I’m mildly torn about this. [Stupid embed isn’t working; link here.]
As GeekTyrant notes, Triss has bright red hair in the Witcher video game, not the books; and they’ve decided to go with the books because this way Anna Shaffer doesn’t have to wear a wig. On the other hand: I know Triss mostly from the video game (the books aren’t bad, but I have a stack as long as my arm to read as it is). And, on the gripping hand, better a dye job for that actress than a fright wig. I have not been impressed with Hollywood’s mastery of brightly-colored wigs lately.
PS: I probably won’t even notice after the first episode, honestly.