Correct me if I'm wrong, but I think the leader of the Ren'dorei is Umbric and he turned purple (literally). Alleria plays the role of teacher for the Velves, but does not lead them.
I disagree.
The identity of the undead lies with their nature, not their appearance. Any race can become a undead. Now the living playable races depend on their appearance and cultural/social reality.
Velves (characteristics that define that they are corrupted by the void)
in the case of Belves and Helves, their political badges with their respective symbols (colors, flags, morals and ethics)
and who knows… even in a future Blizzard can bring Helves with a different model than the Belves creating an accentuated badge as in the case of the stormwind humans and the humans of Kul'tiras
Reducing those characteristics to put other characteristics that their counterparts possess, is to damage their identity as a group. I know that many here like the addition of new customizations for their aesthetic value, but the narrative value is given by the identity.