Tutorial below:

1. First open the yellow tooth picture.

2. Click the icon in the red box, add a hue/saturation adjustment layer, and set the yellow channel.

3. The effect is as follows.

4. I found that although the teeth were whitened, the complexion of the face also changed. I solved it by masking and smearing the teeth with a black brush. Yellow teeth appeared, which was not the desired result.

5. Use Ctrl+I to invert the mask. The final effect is as follows. Teeth whitening is that simple.