Changes in hair color, particularly during puberty, are not uncommon and are generally attributed to genetic and hormonal factors. Hair color is determined by the amount and type of melanin pigment produced in the hair follicles. There are two main types of melanin: eumelanin, which is responsible for black or brown colors, and pheomelanin, which contributes to red and yellow hues. The shift in your hair color from blond to dark brown could be influenced by genetic factors, as certain genes determine the production of different types of melanin. Hormonal changes during puberty can also play a role in altering the pigmentation of hair. The variation in beard color, where most of the hairs are blond despite your dark brown hair, is not uncommon either. Beard color can be influenced by a combination of genetic and hormonal factors as well. It's possible that different genes are expressed in the facial hair follicles, leading to a different color compared to the hair on your head.