|Need I remind anyone that Jake Gyllenhal was the Prince of Persia?|
Katniss's haircolor, while referred to often, really has no meaningful relationship to transitioning the story to film. I promise, promise, promise you that when you are watching the movie, her hair color will be meaningless if her acting is so bad that you can't get into the story. And let's face it, she'll be covered in coal dust and dirt most of the movie anyway.
Ultimately Katniss is a girl who had to grow up quick. She's not hard--she could have been a sweet girl--but she is calloused by her dismal situation. She's a strong person who risks everything for the people she loves. That is exactly the character Jennifer Lawrence played in Winter's Bone. And she rocked it.
So let's all calm down and judge her acting (hopefully she's great and we won't notice her acting at all!) instead of her hair color (in the age where anyone can dye their hair at home).