Headband braids and french braids in general are a great way to conceal roots, because they wrap layers of hair over one another, creating a more uniform look.
Big, overdirected curls
Creating tons of volume at your roots with large, loose curls makes your roots stand up, hiding them.
This is a classic solution, and still a good one. Make like Blair Waldorf and band it over. Using a wide ribbon or a scarf works well, too, and is always chic.
A sleek side-parted bun
In the opposite direction, creating a deep side part and slicking your hair back against your head makes all your hair look darker and makes your roots stand out less in general.
The most direct solution is to powder your roots. Bumble & bumble makes an actual hair powder spray for blond, brunette, and raven-haired touch ups (sorry, titans), but in a pinch you can also use baby powder or face powder to change your shade.