The last time the actress, 26, went to the dark side was in May 2011, just months before getting engaged to her beau, Mumford & Sons singer Marcus Mumford in August.
Prior to that, the star (whose natural hair color is brown) went through a dramatic hair change for a movie. In 2009, she bleached her hair platinum blonde for Public Enemies -- a change which she opened up to Elle UK about.
"I hate my hair. After An Education, I had a small part in Michael Mann's film Public Enemies and he wanted me to dye it white blonde, and it wrecked my hair," Mulligan told the mag. "Literally, ruined it. It was falling apart, fried. So I had to cut it off. I cried."