I Have Consulted With My Doctor About Color Change On

I have consulted with my doctor about color change on my skin, I have darker color around my neck, thinking this was a sign of diabetes I asked my doctor to check my AC1 levels and he said everything is good. I also noticed my knees getting darker, as well under my breast where the bra ends. I am overweight but all of my lab results, according to my doctor come out normal, but I dont know why my skin gets darker around those place and he is not able to answer me