On rule 34 I saw two pictures of him one clothes and one naked, the clothed one shows his balls as black colored, but the naked one shows his balls match his skin color, is that a mistake?
If you're talking about the hyrax Alpha (https://www.furaffinity.net/view/54414264/), I think the clothed bust has his balls contained in a black cloth of some kind; notice what appears to be crumpling at the bottom of the crease between his balls.