Here are the different types of Elites in Halo:CE
Rookie- These guys are blue, and carry the PR or Needler. Even on Legendary, they are CRAP. They can't shoot straight and a lot of the time they will just charge you screaming like a monkey on LSD. To kill them, just give them PR headshots and they will just sit there while you burn 'em. I usually fir 10 shots w/ the PR and then wade in to melee them.
Veteran- These are the red Elites. They still stink, but on high difficulties they can use the Needler or PR to dish out some serious pain. The easy way to kill them is just like a blue Elite, only it takes a bit longer. Also, don't melee them. Once on Legendary I whacked one who was almost dead, and he instantly recovered and bashed me in the face.
Ossoona- These are the spy Elites in silver/light blue. They are almost always camouflaged but they are still easy. They usually have the PR but I think that there is one in AotCR with a sword. Don't waste ammo on them. Just go in and melee until they are down, b/c they usually won't melee back.
Special Ops- The black Elites carrying Plasma Rifles. I have never seen one w/ a Needler but there probably is one somewhere. They throw grenades virtually every few seconds on Legendary, so watch out. They are very hard to kill, but the Plasma Rifle will put them down almost instantly.
Field Masters/Ship Masters- These are the gold Elites. On Legendary they are easily the toughest enemy in the game. They always carry an energy sword, but if you get more than 20m away from them they will whip out a PR. That said, they can jump like a combat form and their sword is a one hit kill on Heroic/Legendary. So watch out. Also, they have overshields and can survive a rocket to the head. Once on Silent Cartographer I literally put two rockets into the Commander before my buddy stuck a nade on him.
So there they are.