Question) At what point should I consider teeth whitening?

I am curious if there are standards for teeth whitening. Are there numerical criteria for normal teeth? Also, is there a difference between the numerical values and the yellowed teeth?

But if it becomes too white, only the teeth will stand out and it might not look good.

I'm curious about to what extent teeth whitening can be controlled or adjusted.

I'm also curious about how often I should do it and how long the effects last once I do it.

I'm curious whether the method of teeth whitening involves applying chemicals to the teeth or covering them with something.

I have no knowledge in this area at all, so I'm curious about many things.