From WENN.com

The brunette admits she came up with the idea of matching her hair colour to that of her literary character because she wanted Salander to look just like the girl in the book trilogy.

Mara explains, ""I had to wear a merkin... They didn't ask me to but I'm naked so much in it (movie) I wanted one. It felt like a little fur bikini... and I had them make it strawberry blonde.""

The actress kept the merkin after the film wrapped and has it at home.