My opinion lies somewhere between jimoblaks and spyderix....
For selecting the area I prefer using quick mask mode over the lasso tool, maybe using a slightly soft brush (but not too soft in this case). I find it's more accurate and less frustrating! Using Paths is another option, although when you convert them to selections, be sure to feather it a little.
As for altering the colors, then I would use one or two of the color adjustment tools, such as Hue/Saturation, rather than painting on a new layer. Again, just my own preference, and you should try out a few of these techniques to see what works best for you.
If you really want to play it safe, create a Hue/Saturation adjustment layer and create a layer mask to isolate the teeth. It's essentially the same technique, but with the advantage that you can always go back to the original image if things go wrong!