A baby's brown hair is usually due to genetics. The production of melanin, the pigment responsible for hair color, starts slowly. A baby's hair color at birth may not be their permanent hair color; it can change significantly over time.
Genetic Inheritance
Brown hair is often genetically inherited from parents or grandparents. Even if both parents have black hair, a child can inherit brown hair due to recessive genes. As illustrated in a Quora discussion (https://www.quora.com/My-son-has-brown-hair-I-and-his-mother-both-have-black-hair-3-out-of-his-4-grand-parents-have-black-hair-one-has-brown-hair-How-can-my-sons-brown-hair-be-explained-genetically), a child's hair color can be explained by the combination of genes from both parents and their ancestors.
Melanin Production and Hair Color Change
The amount of melanin present significantly influences hair color. Light brown hair is often associated with a fair complexion. Light brown hair is genetically acquired most of the time....it's most commonly seen in babies with fair complexion. It can occur due to micronutrient deficiency in very malnourished babies or in some metabolic diseases.20-May-2020
A baby's hair color can change over time. Many examples show babies born with dark hair developing lighter hair later, or vice-versa. (https://www.reddit.com/r/NewParents/comments/17oaiwd/did_you_babys_hair_color_change/, https://www.reddit.com/r/mildlyinteresting/comments/mskkv1/my_daughter_was_born_with_dark_hair_but_now_the/). This is because the production of melanin increases with age. The initial hair a baby is born with can fall out and be replaced by hair of a different color. (https://www.quora.com/Can-a-baby-s-hair-change-from-dark-to-light)
Other Factors (Rare Cases)
In rare cases, nutritional deficiencies or metabolic diseases might affect a baby's hair color. However, genetic inheritance is the primary determinant.
