"It's part of being a girl - we get to change up our thing and confuse everybody," she said of her new look at the premiere of her new film with Jennifer Aniston, He's Just Not That Into You.
If you're looking for a drastic change, experts generally recommended going no further than two shades away from your natural base to make sure the result will still suit your complexion.
And while Scarlett's beautiful new chestnut-brown is clearly far darker than her previous colour, it isn't that far removed from her natural hair which is around the dark-blonde/light-brown area of the spectrum.
Apart from giving you a fresh new look, the great benefit of going from blonde to brown is the amazing glossiness you can achieve because darker locks reflect light more effectively than fair, which makes hair look healthier.
Don't keep flip-flopping through hair colours though, as the peroxide required to go back to blonde can cause extensive damage with repeated use.
If you're looking for a quick makeover to help you survive the winter gloom, try the brand-new Perfect 10 from Clairol Nice 'n Easy, £6.99, which can transform your hair permanently in just 10 minutes.
Shine activating micro pearls infuse colour twice as fast as traditional formulations, with a patented application comb designed to wrap every strand in colour crème.
It covers grey, and the dual conditioning system – the formula contains built-in conditioners, with six, weekly ColorSeal intensive conditioning treatments on top – ensures hair looks glossy for longer.
Try Light Golden Brown to recreate ScarJo's new look.