Just about everyone's bits are darker than their surrounding normal skin tone. It is unique to the person. Nothing to worry about at all.
BTW, at 15 I do hope you are taking precautions in birth control. You will find you'll have more to worry about than your vagina lips if you get pregnant! No offense but do take care!
Different people look different in there private areas, this is just how you look I think. I doubt there would be any way to change it. Every woman at some point wonders if she's normal down there but what is normal anyways? Seeing as we are all unique.