Wowhead’s dressing room uses what’s possible in-game, as I mentioned before but you didn’t read. If you enable NPC options, it allows you to see absolutely everything unique to them, DK’s eyes are separated and not tied to a skin tone like the Dark Ranger customization does, even the skin tone used by Thassarian and Whitemane is separate and doesn’t rely on specific eyes, allowing them to achieve what you’re saying, If what you’re saying were really true, they would have updated their models a long time ago, it’s not a game limitation.
And for very simple examples.
Thassarian: https ://i.imgur.com/0RlRj6O.png
Whitemane: https ://i.imgur.com/u1BkRZL.png