HI, thanks for posting your query
since are slightly over weight you may be having Pseudo Acathosis nigricans which occurs due to obesity, there develops pigmentation at the back of the neck, arm pit , groin
for this weight reduction is the best solution, pigmentation decreases with decrease in body weight.
Depigmenting creams, chemical peeling are not of much help but can be tried ,
you can use melacare
, twice a day but under expert medical supervision as it should not be used for long periods.
Hope this helps you.