Katniss Everdeen in The Hunger Games

For those that want to dress as Katniss for Halloween, cosplay or for a costume party...

Lets start with the book:

Katniss wears a a light green blusa with her mockingjay pin, tawny pants, brown cinto, correia and a thin hooded black jaqueta that falls to the thighs. Her hair is done in a braid down the back. She also wears combat boots.

If you want to go totally accurate you can mess yourself up a bit, a wound just above the eyebrow and flame-painted fingernails (For those that have read the book will know).
And since Katniss has black hair and grey eyes, if you don't have the colour you can dye your hair...