I have seen a lot of teeth whitening tutorials, and this is what I believe is generally the fastest and easiest method.
1. Open the photo you wish to edit.
2. Select the paintbrush tool.
3. Paintbrush Mode- choose "Saturation"
4. Paintbrush Opacity- 70% (Modify this level as necessary)
5. Paintbrush Brush- choose a brush with a soft edge (fuzzy circle), and choose a size that is similar in the size to the smallest yellow tooth.
6. Make sure foreground color is set to black or white. (important)
7. Begin painting over the teeth. Hold the mouse down and do not release until all the teeth have been painted.
tip. Use ctrl+z if you make a painting mistake.