During my teens, I had my 1st girl who will kiss and touch me
everywhere except she won't allow me to see her teats. I didn't
know why, but one night she allowed me to unhooked her bra and I
found her nipples are deep-dark brown. Still I was raw to know
anything.
Many years later, I had new gf and she was more accomodating in
many ways. To my surprise, I found her nipples pinkish. Then I
asked her and she told me (it's circulated among girls themselves)
this: girl must not let guy su*cked her nipples too hard or it will
turn black. She stopped me if I get too intense.
So the lesson is be gentle with girls' asset and take good care of
them.