you guys gotta stop taking these volume cover colours seriously, Oda just uses the colour which is convenient to him at that time, Killingham's eyes are obviously not the exact same colour as his hair, Oda just couldn't be bothered to take out another colour...he's done this multiple times. I mean King was not even dark brown in the volume cover, he was the same skin tone as Zoro. Doesn't mean anything.
The only time I take colour seriously is in high budget One Piece films. The staff really cares for those little details in movies.