Sienna Miller swaps her blond locks for a shock of auburn

Sienna Miller has decided to find out whether blonds really have more fun by trading her signature locks for a fresh auburn 'do, surprising fans on Tuesday night (Sept. 8). The 33-year-old, who rarely makes public appearances outside of promoting films, celebrated the launch of sister Savannah's new collection for British department store Debenhams looking virtually unrecognisable with her dramatic new makeover.

CLICK ON PHOTOS FOR FULL GALLERY: sienna millerSienna Miller unveiled her new auburn locks on Tuesday evening.

Looking every inch the style icon, Sienna complemented her new locks with her effortlessly cool personal style. The star rocked an oversized grey sweater from her sister's new collection, Nine by Savannah Miller, which she teamed with black skinny jeans and accessorised with a pair of tan suede peep-toe mules.

The American Sniper star hasn't just ditched her blonde locks in favour of red hues because she fancied a change – the bold new look is part of her makeover for her latest role in the upcoming movie The Lost City of Z, which she's currently filming in Northern Ireland.

However, Sienna made sure to take time out of her hectic work schedule to come to London and support her sister at the glamorous launch party held at London's trendy Ham Yard Hotel.

savannah millerSienna supported Savannah, who is launching a new collection for Debenhams.

Savannah recently revealed exclusively to Hello! that she turned to Sienna for advice when she was creating her new designs. "We do that anyway because we're sisters and best friends," she told us. "I did ask for her opinion about certain things. She always is in agreement and I really respect her style."

The mother of three also admitted that while the stylish siblings talk about fashion, it's something they rarely talk about nowadays.

"We don't really have time for that, being mothers," she admitted. "Neither of us spend hours discussing our style at all, it's more about what we like and wearing things we like."

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