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.
Recently rewatched Dinosaurs (the 90s puppet sitcom). They do an excellent job of pointing this out, especially in the episode "dirty dancing"
Earl immediately points out how TV sucks because they make one hit TV show with a premise and then a 50 more of the same after. Then the whole episode is about Fran (his wife) nagging him to be a good father to their son, and him being useless and lazy dad stereotype.
Great show. So much still relevant.