Invisible Elites, These guys are always invisible and are the weakest too. You will always see them with a glowing plasma sword. Just look for the hovering glowing sword, and you will know it's one of them.
Blue Elites, These are the low-level ones. Out of all Elites, this is the most seen one. They carry a plasma rifle with them and can take a very good beating. So be prepared for a battle.
  Purple Elites, More improved version of the blue Elites. They of course have more battle tactics and carry a wide array of Covenant weapons.
Maroon Elites, A rarely seen Elite, which is very hard to kill. Once again they as well carry a wide array of weapons and will give you a very good beating.
Gold Elites, Very powerful Elite, sort of the leader of them all. They will always carry a plasma sword with them.
Black Elites (Special-Ops), The best of the best. These guys are not to mess with. They are seen carrying any Covenant weapon including grenades. Combat wise, they are very quick and deadly.