Known for her beachy tresses and bleached-out colour it was a welcome surprise to see Olsen with dark and sleek, glossy lengths. Her colourist Josh mixed the colour saying ‘it’s such a beautiful, warm brunette shade.’

For anyone wanting to achieve a similar look at home he recommends using John Frieda Precision Foam Colour in 4BG, £6.66 (boots.com). However, he warns that ‘when making a drastic colour change, it’s always best to seek out the help of a professional. Going from blonde to brunette is less harsh on your hair than going from dark to light, but you still need to see a pro.’

It wasn’t just team ELLE excited to see Olsen’s new colour, her longtime hairstylist Mark Townsend was pretty psyched too. 'I was away for the weekend so I raced over to her apartment to see it [the following] morning. I think she was just ready for a change.'

'I don’t think Mary-Kate or Ashley ever make really rash decisions. They have really clear ideas. [No one] is more aware of their own sense of style.'

A sense of style is important, so it’s key that if you want to go brunette like Olsen you find a shade that suits you, not just a carbon copy of the designer’s. 'You need to look at both your skin and eye color to determine whether you're 'cool' or 'warm'. People with 'cool' tones have darker eyes and skin that's either pale with pink undertones, olive-toned, or dark skin. The best hair colours for you are dark chocolates, light ash, or dark champagne.

'If your skin has golden undertones and you have golden brown, green, or hazel eyes, you're probably 'warm'. You look good with golden brown or honey hair, as well as brunettes with copper undertones', advises Josh. If you’re in the cool category and you want to try Olsen’s colour, don’t be put off just yet. 'It suits those in the "warm" family but make-up can make almost anything possible!' says Josh.

Of course we don’t all have access to celebrity stylists like Josh, so if you do want to book in with your colourist Josh advises taking a picture of the colour you want. 'A photograph is worth a thousand words. Don't bother to articulate shades - everyone has a different view of what warm means. Show the picture!'

Once you’ve taken the plunge and gone brunette look after it with a colour glaze once a week. Josh recommends John Frieda Brilliant Brunette Liquid Shine Shock Perfecting Glosser, it will keep your colour looking vibrant. I would also recommend getting regular trims - if the ends of your hair look damaged and uneven, it will draw attention away from your beautiful colour.'

Brunette hair reflects the light so well which is why Townsend wanted to showcase Olsen’s new colour with a sleek style, rather than the beachy texture we’re used to seeing on the star.

'I think this was the first time I’ve really pushed Mary-Kate,' Townsend said of Olsen’s poker straight, centre parted CFDA hairstyle. 'I wanted [her hair] to reflect the light.”Townsend used hair oil before straightening the mid lengths; he ensured the style stayed smooth with a few spritzes of hairspray on a finishing brush. 'It takes out any flyaways around the part or hairline,' he says.

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