I can understand your concerns,the thing is that it all depends upon the skin Color and type aswel if your complexion is brown or dark then it's something natural for the private parts to be darker.
Similarly it also depends upon the hormonal
imbalance like for example in pregnancy
due to hormonal changes these area can get darker than usual.
So as long as you keep the areas clean and there is no other associated illness or symptom you can leave it alone,because in my knowledge there is no specific medication or a procedure which can change the Color.
I hope that will help.