There is indeed an in-universe explanation. It’s the exact same explanation that allows dwarves to have varying hair and eye color. These are literally all the same thing. They were apparently created with genetic mutations similar to humans which govern melanin production in various body parts. Hair, eyes, and skin are all the same in this respect. If you want to accept two of those and deny the third simply because your mind can’t get over that hurdle then that’s a you problem, not a problem with the explanation.