Photoshop Tutorials: Whiten and Brighten Teeth In Photoshop
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Step 5: Select The Brush Tool
Select the Brush tool from the Tools palette, or press B on your keyboard to quickly select it:

Step 6: Set White As Your Foreground Color
Make sure you have white selected as your foreground color, since we're going to be painting with white. To quickly set it to white, first make sure you have the Hue/Saturation layer mask selected in the layers palette, and then press the letter D on your keyboard, which will set your foreground color to white and your background color to black:

Step 7: Paint With White Over The Teeth To Restore The Whitening And Brightening
With our brush selected (you'll want to use a hard brush for this), the layer mask selected, and white as our foreground color, all we need to do now is paint over the teeth with our brush to bring back the whitening and brightening. You'll probably need to change the size of your brush as you're painting, so to change your brush size on the fly, press the right bracket key on your keyboard to increase the brush size and the left bracket key to decrease it. You'll find the bracket keys to the right of the letter "P".
Simply paint over the teeth with the brush to reveal the Hue/Saturation adjustment layer on those areas and make them whiter and brighter. You may find it easier to zoom in on the teeth, as I'm doing here:

If you accidentally paint over an area you didn't mean to, just press the letter X on your keyboard to switch your foreground color to black and paint back over that area. Then press X again to switch back to white and continue painting.
For some areas, you may also find it easier to paint with a lower opacity brush. You'll find the Opacity option in the Options Bar at the top of the screen. A lower opacity of around 20% is sometimes helpful to avoid over brightening teeth in the darker corners of the mouth.
Continue painting over the teeth until you've whitened and brightened them all:

Now that I've finished working on his teeth, I'm going to use the same Hue/Saturation adjustment layer to fix the woman's teeth as well, and I'll do that next.