Other forms: understatedly
Anything that's understated avoids extra decoration or embellishment, like your best friend's understated prom dress, which is somehow more elegant and classic than anyone else's outfit.
If your decorating style includes loud colors, lots of knick knacks, big chandeliers, and extra pillows everywhere, nobody is going to describe it as understated. Use this adjective for a restrained, tasteful, and simple style, like your grandma's perfectly understated string of pearls or an actor's beautifully understated interpretation of Hamlet.