I think that the pigment changes will take care of themselves, without treatment. There is, in any case, little effective treatment for lightening of skin after inflammation. Resolution takes several months.
Having said that, I add that I am not familiar with the materials used by implant dentists. It may be that they use a particular chemical which causes this problem (though no one has ever consulted me about such a problem.) You really ought to ask the dentist about the natural course of lightening in other patients it's happened to. If you're told that it never or almost never happens, then you can feel comfortable at just waiting it out. Further sun exposure should not prevent such improvement.
Take care.
Dr. Rockoff