Whiten Teeth
Select just the teeth with the ‘quick mask’ (located under the color swatch in the tool bar).
Go back to standard mode and inverse the selection. Choose the ‘Hue/Saturation’ adjustment layer.
Edit > Yellows from the dropdown box. Then desaturate to -100 or desired.
Edit > Master and add some brightness to remove the grey from the teeth. “Ok” out of the dialogue box.
Change the layers ‘opacity’ if it’s too white. Then you can paint black, white or grey over the darker teeth to change those as well on the layer mask.