During puberty, new growth, different gene expression, and a different hormone mix means that secondary hair growth can be a different and darker shade than the stuff on the head. For men especially, the body hair will be darker than hair on the scalp. At 13, my pubic hair was very fine, very blond. I had nothing on my chest. Today, 30 years later, I have some hair on my chest, a good deal on my belly and legs, anywhere from light brown to dark.