The Secret Diary of a Call Girl actress showed off her darker brunette barnet as she attended the opening night of American Psycho: The Musical in London's West End yesterday (12 December).
Billie was on hand to support her pal and Doctor Who co-star Matt Smith who stars as the show's lead character Patrick Bateman.
The twosome threw their arms around each other as they posed at the musical's after party, with Billie sporting a toasty pillar-box red coat.
The 31-year-old starlet has kept her tresses a warm blonde shade for years, so we were pretty surprised to spot her rocking such a dark hue.
Billie opted for a mid-toned brown with auburn and caramel highlights for a natural, multi-tonal effect, styling her locks in tousled waves.
To go with her new bonce Billie wore hardly any make-up, skipping the foundation and lippy and just slicking on a few coats of mascara.
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