The image I am using for this tutorial was downloaded from StockExchange.
Ok, so the first thing we do is to load the image into photoshop. (I write these tutorials assuming you also use photoshop. Shortcuts, buttons and functions might be different if you are using something else)
1. Once you open the photo, make a copy of the background layer by pressing CTRL-J, or right-click and select duplicate layer.
2. Enter quick-mask mode by pressing Q
3. Select the brush-tool by pressing B and adjust the hardness of the brush up a bit so that it will make nice and clear edges.
4. Zoom in on the persons teeth and start painting the teeth using black as the foreground color. You will see that the teeth will be covered with a nice red color.
When you are finished, they should look something like this:
5. Press Q to exit quick mask mode and you will see a nice selection of the teeth, looking something like this:
If you see your selection covering more than just the the teeth, go back into quick mask mode and select white as the foreground color (by pressing X) and then paint over the area that you dont want. You will then see the red color go away. Check your result by exiting quick mask mode by pressing Q.
6. When you are happy with your selection and have exited quick mask mode, press CTRL-SHIFT-i to invert your selection.
7. Create a new hue & saturation layer as a new adjustment layer by choosing layer on the toolbar. Then go down where it says 'New Adjustment Layer' and choose Hue/Saturation.
A dialog will pop up, looking like this:
Make sure the box where it says' Use Previous Layer...' is checked, then click OK.
8. Now adjust the sliders for the hue & saturation layer. For this particular image i'm using, i have used the following settings:
You might want to zoom out a bit when doing this as it will make it easier to see the result of what you are doing. When you are happy with the adjustments you have made, save your image, you're finished. The results for this image wont be really astonishing since she had very nice teeth to begin with, but here is my result (slightly down-sized) :