If you’re gazing into the eyes of a newborn baby, more often than not you’ll see they’re some brilliant shade of blue, whatever the color of their parents’ eyes. This happens because, when a child is born, its eyes are still building up their store of a protein called melanin. The more melanin that collects, the darker the iris will become. Not only that, melanin itself becomes darker with exposure to ultraviolet light. At around six months old, a baby’s eyes will usually reach the color they will retain for the rest of their life.