You'd be wrong. Seeing the same zombies with different skins definitely adds variation. And you stop seeing the glaring similarities.
Madmole's logic is also wrong. Minecraft works because it's ONE zombie skin, which our brain begins to discard. Seeing 15 zeds and 9 are alike is more distracting than seeing 15 of the same.
He is right that they have better things to do tho.
Having code change the tint of a shirt material on spawn also wouldn't add any real overhead btw.
Been there, done it, just saying.
Fight me.