Add a Hue & Saturation adjustment layer by clicking on the Create a new fill or adjustment layer icon in the layers palette and selecting Hue/Saturation.... Move the Saturation slider all the way to -100 and click OK.
This has turned your image into a gray scale image, but we're going to fix that in Photoshop by pressing Ctrl + i (Command + i on the Mac). What happens is that the mask (filled with white), which was automatically added (and active) when we added the adjustment layer, was inverted and is now filled with black. A black mask means that none of the desaturation will be applied to the image underneath that layer.
Rename this adjustment layer to Teeth by double clicking on its name in the layers palette.

You're now going to paint on the mask to reveal that particular area that needs desaturated colors, which are the teeth.
First press the letter D on your keyboard to make sure that the foreground color is white:
Make sure before you do that, that your mask is still active; it will have a double border and there will be a mask icon in front of your layer.

Continue by zooming in on the teeth to about 400%.

Grab the Brush Tool in the tool bar and in the options bar click on the button marked with A.
Enter a Master Diameter of 5 px and a Hardness of 0%:

Make sure that the Opacity is set to 100% for this brush in the options bar of Photoshop.
Now comes the tricky part; paint over the teeth that need to be desaturated, but avoid the gum and lips. Don't paint on the darker areas between the teeth, but only focus on the teeth.

You made a mistake? No problem, press Ctrl + Z (Command + Z on the Mac) to undo your last step. But more important; you can always fill the painted (white) areas in your mask with black again by painting on them using a black brush. A quick way to do is by pressing the letter X on your keyboard. This will switch the back- and foreground color. Needless to say that you have to press X again (switches the foreground color to white again) if you want to continue where you left off.

Return to the Hue & Saturation adjustment layer when you're done by double clicking on its thumbnail

Set the lightness to +40 and click OK:

 

Set the Opacity of this layer to about 40%:

 

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You might consider to use a higher value for opacity, but be careful that you don't make it too high; the result will look unnatural. That's why it's so important to turn this layer on/off (click on ) to see the difference.

Tip: You can color the layers in the layers palette by using that same icon; right click on the icon (Control + click on the Mac) and select the preferred color in the window that pops up (in this example Yellow is selected):

 

Lock the Teeth layer.

You might have noticed that one tooth seems to be a slightly turned the wrong way:

Because of this we also see a slightly distracting highlight on that same tooth. Let's see how we can fix this in Photoshop. Add a new layer by clicking on the Create a new Layer icon in the Layers Palette.
Rename this layer to Tooth.

Zoom in to a value between 500%-800% and fix the tooth with the Clone Stamp Tool . Select a Clone Stamp brush with a size of 2 or 3, hardness of 0% and use an opacity of around 30% (can all be selected in the options bar). To set the source (like we did in our previous step), use the area to the right of the highlight as seen in this screenshot:

Lock the Tooth layer.

 

 

It's not as difficult as it seems. Just remember; be patient and don't try to take huge steps, meaning that you shouldn't go for a higher opacity than 30% or use a larger brush just because it "takes so long". Also remember that you're working on a separate layer so if you're making a mistake you can always go back.