Loads of server have had it, some have had other concepts too like on Archon's you'd have the Blue Aura for Super, then Red Aura for Elite, remember Scorp having to Reboot for some Super/Elite bosses that was impossible to kill lol, pretty sure Bon had a variance of mobs on 5h that went all the way to Red which was like super elite verison of mob with really good exp.
Marble just had it the cleanest imo, It was like a buzz to get a level 7 mob in PB / Red cave such a fat bulk of exp and chance of a really good drop can't remember getting that sorta feel from any other server concept of this.
Originally it was just 1-7 changing colours for the leveled mobs but im sure somebody complained to Jamie about being colour blind or something so implemented text into it " Level 7 WingedOma " But it still kept the colour's in too i think.