Mine ain't.

Assuming you haven't seen other people's nipples and maybe that's why you think they are pink. Nipple colour is influenced by skin tone/melanin & genetics. Caucasian people have less melanin, so their nipples tend to be more pink. Most people who are not white have light to dark brown nipples.

Idk about the science behind it though, other mammals don't have coloured areolas as far as I know.