The American Sniper actress has been on our 'we want her hair colour' list for, like, ever, with most of the Reveal office craving after her perfectly toned blonde locks.
But on Tuesday night (8 September) she showed off a very dramatic new look – and sorry, Si, but we're not 100 percent sure about the red...
Heroes star Ali Larter went from blonde to a warm red just last week for her role as Claire Redfield in the new Resident Evil: The Final Chapter movie and we loved her gorgeous new hue.
But Sienna's colour job just isn't quite hitting the mark for us.
We don't know if it's due to the fact she was wearing very minimal make-up as she was snapped at the launch of sister, Savannah Miller's new Nine by Savannah Miller fashion line for Debenhams at The Roof Terrace at the Ham Yard Hotel in London.
Or maybe it's just because copper is not a patch on her trademark blonde, highlighted tresses.
To be fair, it does suit her as it brings out her freckles and intensifies her blue eyes. But, and correct us if we're wrong, doesn't it age her a little bit?
Sienna's hair was coloured for her role as Nina in The Lost City of Z, which she's been shooting in Northern Ireland alongside Son's Of Anarchy actor, Charlie Hunnam.
So maybe it's just a temporary thing.
We hope so. Bring back the blonde!
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