Alba, 31, is set to star again as stripper Nancy Callahan in the scheduled 2013 release, "Sin City: A Dame To Kill For," alongside former co-stars Mickey Rourke, Rourke and Michael Madsen.
Alba showed off her new haircut in a series of photos on her Instagram and Twitter accounts.
"Stage 1 -ok it doesn't look that different but it's a process," Alba wrote as she started her new haircut.
"Stage 2 of #SinCity2 Nancy hair," she later wrote as Alba's brunette locks took on a blonder tone.
A third photo of Alba's new haircut shows darker roots with blonde ends.
"She got roots that Nancy #sincity2," the actress wrote on her Instagram account.
The "Sin City" sequel is slated to be released October 4, 2013, and is the sequel to 2005's "Sin City," the film based off Frank Miller's graphic novel. "A Dame To Kill For" is also adapted from one of Miller's graphic novels.
Miller and "Machete" director are both writing and directing "Sin City: A Dame To Kill For," according to the film's IMDB page.
This is how the website summarized the "Sin City" sequel:
"In the dark bowels of Sin City, Dwight plans to have his vengeance against the woman who betrayed him, Ava Lord, while Nancy is trying to cope with Hartigan's death."
The film is currently in pre-production as of Aug. 7, according to the IMDB page.
"Sin City" fans are anticipating the sequel and website comicsalliance.com posted a teaser poster for "A Dame To Kill For" and spoilers for the story here.