If you’re referring to Lady Alistra, the only closely related red eyed NPC to death knights, we don’t even know if she’s a death knight. And we effectively know nothing about her, much less why her eyes are a different color. Suffice to say, she probably wasn’t raised in the same manner as us. It may be possible that she is, or is closer to a second gen Death Knight, who don’t necessarily die, and just swear allegiance to the Lich King (see Lord Thorval, etc.).
Plus, if the argument is “eye colors to match spec,” then she runs counter to the whole concept, as she is associated with/in the Unholy wing of Acherus, and can frequently be seen demonstrating Unholy spells like Corpse Explosion.