This is the best way to whiten teeth in Final Cut Pro. Let's cut the fluff and hop into it. We got very lucky with the arrival of auto masks in FCP, because they make specific color correction like this so much easier.
Step 1: Auto mask the teeth
Open the inspector, go to your color inspector, and add a Color Wheels correction. See the little person icon with stars around his head? Click it and select Auto Mask. FCP automatically identifies things in your shot: hair, mouth, lips, teeth, eyebrows, clothing, foreground, background. Select teeth.
The red overlay just shows what's selected (no, your subject doesn't end up with red teeth), and the fact that it knows the difference between gums and teeth is genuinely impressive. Click Add Auto Mask, and if you're stacking multiple masks on the shot, give this one a label. Press Done to lose the red.
Step 2: Desaturate, don't bleach
Take the global saturation slider, which now affects only the teeth thanks to the mask, and lower it. Don't go too far or it looks unnatural, like a toothpaste commercial gone wrong. Zoom into the viewer (click it and press Command Plus or Minus) so you can see what you're doing, and find a natural amount.
Step 3: Brighten and blend
Bump the highlights exposure slider up ever so slightly to add brightness. Then add a tiny bit of blur to the mask, just enough that any harsh edges disappear. Too much and it undoes your work; a touch just softens the transition.
Toggle it off and on and look at that difference. It looks fantastic, it took two minutes, and nobody's teeth glow in the dark. Have a fantastic day!
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