Ok his hair color is never explained in the novel, but I think the main reason his human hair color and puppy form hair color doesn't match is bc of his pheromone levels. We know that Heeseong's pheromone level is super low (like around 5 or smth) so that could explain why Heeseong's human form doesn't have white hair since it's not a normal hair color irl. But for dogs, white isn't a recessive hair color.
And Yoo Chi-young's human hair matches his wolf form's hair color bc of his strong wolf blood. And every other beastmen has a normal level of pheromones, so their animal form usually matches their human form's hair color.
Okay the puppy part is explained but WHY is his coat white but he's a brunette??? I've been annoyed abt it from the beginning