The Enchanted actress said she only started landing lead roles when she dyed her blonde hair.
The 34-year-old spent years playing small parts in films such as 1999's Drop Dead Gorgeous and 2000's Psycho Beach Party, while a role in 2002 film Catch Me If You Can failed to ignite her career.
She changed her look in 2004 and has since been nominated for an Oscar for 2005's Junebug and a Golden Globe for last year's Enchanted.
She told InStyle: "I've had red hair for the past four years. Before that I never thought of being anything but blonde.
"There was a definite shift in my life when I decided to go red. When you're blonde, people associate it all with being flirty. With red hair, suddenly you're quirky."