Dermatologists Dr. Cameron Rokhsar and Dr. Lindsey Bordone spoke to Daily Mail Online to explain why your penis and nipples are a different colour and the answer is less obvious than you think.
It's all about sex hormones. Hormones such as testosterone and estrogen manage the activity of our melanocyte cells - the cells responsible for releasing melanin, the protein pigment that affects our skin colour.
During puberty, we can expect an increase in cells - that's why our nipples and Johnson darken during our teenage years.
“Hormones regulate the way melanocytes produce their pigment," Dr. Rokhsar told the publication.
"Those can have an effect on the way the skin in that area pigments, so generally that area is darker."
“When girls are young, their nipples are light,” Dr. Bordone added.
“And as they get older, hormones have an effect and the nipples darken. It’s very similar for boys.”
Private parts aside, this is also explains why your hair is lighter as a kid.
In rare cases, the darkening of areas can also signal health problems such as diabetes.
“Your skin can darken in certain areas when your sugars are very high – and you’re developing diabetes,” she mentioned.
If in doubt, consult a medical professional.