You can try tinted dry shampoo, I've used Batiste's blonde and dark brown versions - I actually used it for a
The Last Jedi Luke cosplay to make my blond hair darker and it was pretty effective. It washes out easily and since it's dry shampoo, and you can control how light or dark your hair goes by how much you spray. One note, though, it's basically like tinted baby powder, so it can get on your clothes or hands, so I wouldn't use it until after you are dressed in whatever you're going to wear, particularly if you have any shirt that has to go over your head. I never had any problems with it getting on my clothes while spraying or after, but I did try to be careful when I sprayed, and I also used hairspray afterwards, which probably locked in the powder. If you spray too much too close to your head, it will color your skin too, but it wipes right off. I would hold my hand over my forehead to help control that, then just washed my hands after applying.
There's blonde, brunette, and dark versions, and they're easy to find at a lot of different stores. Not sure where you live, but they're available at a lot of major pharmacies, big box stores, and beauty supply stores, plus you can order them online. I'd guess the "beautiful brunette" would probably be the one you'd want. Ewen's hair looks reddish at times, and Batiste says red heads can use the brunette one. But I've also mixed the blonde and dark versions to get different shades, so you could also try that.
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