Armor customization

Armor Permutations is a feature in Halo 3, allowing the player to aesthetically customize their multi-player avatar (the armor doesn't have any effect on gameplay).

Introduction
Permutations allow a player to customize his/her multiplayer avatar (either a UNSC SPARTAN-II or a Covenant Elite) by choosing different helmets, shoulder pads, chestplates, and detailed colors (primary color, secondary color, and armor detail). All permutations are aesthetic only and do not change actual gameplay in any way.

They were originally discovered in the foremost release of the Halo 3 Beta in internal hexadecimal codes by modders and were later confirmed to provide high levels of player customization by GamePro's June magazine issue. Calculated together, there are 187,947,000 possible combinations for permutation pieces for the player to choose from.

Armor Permutations are available only in Halo 3 (at the beginning of the game, the only parts unlocked are the Mark VI Mjolnir Armor, Mjolnir Mark VI CQB Variant, and the Elite Combat and Assault variants, as well as the Commando shoulders.), armor is unlocked through achievements in both the Campaign and multi-player modes of Halo 3 or by finding the hidden skulls.

Bungie has confirmed (several times) that no new armor permutations will be available.

I mean there are no, and never will be new Elite Armor permutations or permutations of any kind in Halo 3 the interactive entertainment experience from Bungie Software Products Corporation.- Bungie Weekly Update 7/25/08

SPARTAN Armor Permutations
There are a wide variety of SPARTAN II armor permutations, with 11 helmets, 8 shoulder pieces, and 8 chest pieces for a total of 6,144 different SPARTAN armor permutations: one helmet, two shoulder pieces, and chest piece per permutation, with the exception of the Rogue, Mark V, and ODST, which do not have either chest nor shoulder pieces, and Security, which does not have a chest piece.

Helmets

 * Mark VI: Unlocked by default.
 * CQB: Unlocked by default.
 * EVA: Unlock the Campaign Complete: Normal achievement.
 * EOD: Unlock the Campaign Complete: Legendary achievement.
 * Hayabusa: Collect 12 Skulls in Campaign. Hidden skull not needed.
 * Security: Unlock 1000 gamerscore in Halo 3.
 * Scout: Unlock the Used Car Salesman achievement.
 * ODST: Unlock the Spartan Graduate achievement.
 * Mark V: Unlock the UNSC Spartan achievement.
 * Rogue: Unlock the Spartan Officer achievement.
 * Recon: Bungie employees, winners of tournaments, and those that impress (or amuse) Bungie in any other way or doing helpful comunity service that benifits Bungie.

Shoulder Pieces

 * Mark VI: Unlocked by default.
 * CQB: Unlocked by default.
 * EVA: Complete The Ark in Campaign on Normal or higher difficulty.
 * EOD: Complete the level The Ark on Legendary difficulty.
 * Hayabusa: Collect at least 9 skulls.
 * Security: Earn 750 of 1000 gamerscore on Halo 3. Due to inconsistency, this armor permutation is not always given at 750. There are some reports that the Security Shoulders need a minimum of 500- 995 gamerpoints, even though the official guide to Halo 3 lists the requirements as 750 (could also be total metagame points). Even then, the armor permutation may be unlocked, and then re-lock itself after a time, as has been reported. The only way to keep it permanently is to get all the achievements. This permutation may need the player's model to have accessed online to unlock it. A guaranteed way to unlock security is to gain all 49 achievements. For some players it can be unlocked, then lock itself, but to re-unlock you it you need to change to Mark VI then change it back. It can also be unlocked for getting 25 of the original achievements, but then disappears once you log out.
 * Scout: Unlock the Mongoose Mowdown achievement.
 * Recon: Bungie employees, winners of tournaments, and those that impress (or amuse) Bungie in any other way or doing helpful comunity service that benifits Bungie.

Chest Armor

 * Mark VI: Unlocked by default.
 * CQB: Unlocked by default.
 * EVA: Complete The Tsavo Highway on Normal difficulty or higher.
 * EOD: Complete The Tsavo Highway on Legendary difficulty.
 * Hayabusa: Collect at least 5 skulls in campaign.
 * Katana (Add-On): Have all skulls and get 1000 gamerscore. It is the Hayabusa chest piece with a Katana sword.
 * Scout: Unlock the Too Close to the Sun Achievement, although has been reported that it may appear later sometimes.
 * Recon: Bungie employees, winners of tournaments, and those that impress (or amuse) Bungie in any other way or doing helpful comunity service that benifits Bungie.
 *  Flaming Helmet: A popular misconception is that the Bungie Flaming Helmet is actually a helmet, but in fact, it is a piece of fire on the collar. Thus, it is a chest piece. It is the Bungie Chest Piece that allows the flame on the head which can be used with any helmet and shoulder piece, the only way to get it is to get hired, get fired.

Sangheili / Elite Armor Permutations
There are a wide variety of Elite armor permutations, with 5 helmets, 5 shoulder pieces, and 5 chest pieces for a total of 625 different Sangheili armor permutations: one helmet, two shoulder pieces, and chest piece per permutation.

Helmets

 * Combat: Unlocked by default.
 * Assault: Unlocked by Default.
 * Flight: Unlock either Campaign Complete: Heroic or Campaign Complete: Legendary achievements.
 * Ascetic: Unlock the Steppin' Razor achievement.
 *  Commando: Unlock the Overkill Achievement.

Shoulder Pieces

 * Combat: Unlocked by default.
 * Assault: Unlocked by default.
 * Flight: Complete the level The Ark on Heroic or Legendary difficulties.
 * Ascetic: Unlock the Overkill achievement.
 * Commando: Unlocked by default after Auto Update 1 was applied on February 19, 2008. At first, the Steppin' Razor achievement was needed to unlock the piece, but a bug prevented this. Some people reported getting the shoulder pieces, but then they re-locked themselves after a certain amount of time without using them.

Chest Armor

 * Combat: Unlocked by default.
 * Assault: Unlocked by default.
 * Flight: Complete the level Tsavo Highway on Heroic or Legendary difficulties.
 * Ascetic: Unlock the Up Close and Personal achievement.
 * Commando: Unlock the Triple Kill achievement.

Trivia

 * There was a prank pulled on April 11, 2008, on the 1UP show, which involved a falsified armor permutation called the Assassin Armor.
 * The EOD armor chest is surprisingly similar to the Mark V chest plate. Many people have noticed this and wear the chest plate with the Mark V helmet and the Mark VI shoulders to re-create Mark V armor from Halo: CE.
 * There was originally going to be an Arbiter Armor Permutation but was taken out before Halo 3's release.
 * Before the release of Halo 3 Beta, it was speculated that there would be Brute Armour Permutations.
 * Elite armor is normally considered harder to headshot from behind compared to Spartan Armor although easier from the side.
 * It is advised to us a smaller helmet and a thinner chest and a small flat shoulders to avoid geting sniped or hit with a sniper. Larger helmets can give off your position if you are hiding and they give larger and easier targets for snipers.
 * It was recently discovered that the neck of an Elite(regardless what type of permutation used) is completely immune to an form of damage. In addition, the projectile will simply pass through the neck as though the neck wasn't even there.