This is a terrific video on how to whiten yellow teeth in Photoshop, taught by Link. First, he zooms in on the teeth and uses the quick selection tool or magic wand. This is a pretty easy task. You just have to click and drag. If you put too much of the gums, you can hold down alt, then drag the areas you don't want. Make sure you use a small brush when using these modifications. All you have to do then, is go into the heat saturation and select shift-control-I, which inverts the selection. Then you add a mask and he tells us to make sure your teeth are not too gray and not too white. Then back into the hue/saturation and you're good to go using all of his tips! This is a great way to learn how to use Photoshop for something we all want to fix!