Colour is determined by hormonal changes. Go google it. After birth, nipples will darken so as to help the baby find the nipple for breast feeding.
Colour is also influenced by race. Can you imagine 4 naked Indian ladies with pink nipples? If they start singing, their group name will be Blackpink. How you like that?
Yes, areola color will start darkening during the pregnancy period. For those Chinese ladies i came across, darker skin usually have darker nipples.
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19 Years in SBF and still counting. Dare not count the $ spent though.