1. Let there be light.
"Most women pick a brunette shade that's too dark, so they're left with black hair. They don't realize how dark 'dark brown' can come out," says White. So unless you're jonesing for Katy Perry's licorice hue, it's more face-flattering to choose a medium brown.
2. Hit the right tones.
If a silky, chocolatey-brown shade (think Katie Holmes or Mila Kunis) is what you're after, pick a color with "ash" in the name. Sure, it might look boring and even muddy on the box, but it will get rid of any red tones. On the flip side, if your already brown hair is mousy and dull, or you're starting with blonde hair, choose golden or mahogany tones that add richness and warmth (like those on Zooey Deschanel and Sandra Bullock).
3. Pump it up.
In addition to looking like a helmet, "flat, one-shade hair lacks sex appeal. Dimension gives the illusion of movement, enhances layers, and makes hair look younger," says New York City salon owner Sally Hershberger, whose own medium-brown hair sparkles with golden ribbons. In a salon, pros mix three or four colors to get a multitonal effect; at home, tryMadison Reed Radiant Cream Color, which Hershberger and her top colorists helped formulate so you can reap similar multidimensional benefits on your own.
4. Consider your hair's past.
Your ends—which tend to be damaged and soak up more color—can come out darker. So apply to your regrowth first (if you colored four weeks ago, it's the top half-inch; if it's been eight weeks, it's one inch), then to your lengths for the last 10 minutes. When touching up weeks later, says Corbett, add a bit of water to the ends and mush it around; diluting it will prevent dull, heavy color.