Point to Point select teeth

adjust tooth color with RGB

Colors Adjust Red/Green/Blue

whitening teeth   Add more blue

whiten & brighten Increase Red & Green & Blue equally, then slowly increase your blue while watching your previews

whiten not lighten  Decrease all three colors equally ( teeth become temporarily darker) gradually increase the blue while watching the preview.  Once satisfied with the whiteness, increase Red Green and Blue equally to return brightness to normal.

Teeth stain & are darker up near the gums.
They become lighter & more translucent down near the tips.

Tooth color can be smoothed and more evenly blended with the Clone tool ( best choice) or Smudge tool (okay for small areas) Set your brush with lower Hardness and lower Opacity and paint away.

For tougher jobs:  Use feathered selections (carefully avoiding lips) to apply corrections with the Red/Green/Blue.