Armor customization

"Armor Permutations" is a feature in Halo 3, allowing the player to aesthetically customize his/her multiplayer character.

Introduction
Armor Permutations allow a player to customize his/her multiplayer character as either a UNSC SPARTAN-II or a Covenant Elite, and change between different helmets, shoulder pads, chest armor, and detailed colors (primary color, secondary color, and armor detail). All permutations are aesthetic only and do not change gameplay (no bonuses, increased hitboxes, etc).

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

Armor Permutations are unlocked by accomplishments in either the campaign or multiplayer modes of Halo 3.

SPARTAN Armor Permutations
There are a wide variety of SPARTAN armor permutations, with 11 helmets, 8 shoulder pieces, and 8 chest pieces for a total of 6,144 different SPARTAN armor permutation combinations: one helmet, two shoulder pieces, and chest piece per permutation. This excludes color variations.

Helmets

 * Mark VI: Default Unlocked.
 * CQB: Default Unlocked.
 * EVA: Complete Campaign on Normal, Heroic, or Legendary difficulties.
 * EOD: Complete Campaign on Legendary difficulty.
 * Hayabusa: Collect all 13 Gold and Silver skulls.
 * Security: Get all achievements in Halo 3.
 * Scout: Complete "Used Car Salesman" Achievement.
 * ODST: Complete "Spartan Graduate" Achievement.
 * Mark V: Complete "UNSC Spartan" Achievement.
 * Rogue: Complete "Spartan Officer" Achievement.
 * Recon: Bungie Employees and winners of tournements/double exp weekends.
 * Flaming Helmet: Bungie Employees only.

Shoulder Pieces

 * Mark VI: Default Unlocked.
 * CQB: Default Unlocked.
 * EVA: Complete The Ark on the Normal, Heroic, or Legendary difficulties.
 * EOD: Complete The Ark on Legendary difficulty.
 * Hayabusa: Collect at least 9 skulls.
 * Security: Get 750 of 1000 gamerscore on Halo 3|Due to inconsistency, this armor permutation is not always given at 750, whereas there are some reports that the Security Shoulders require a minimum of 800 gamerscore, but the official guide to Halo 3 lists the requirements as 750. It has been suggested that meta-game achievements and Marathon Man are required.
 * Scout: Complete "Mongoose Mowdown" Achievement.
 * Recon: Bungie Employees and a selected few players.

Chest Armor

 * Mark VI: Default Unlocked.
 * CQB: Default Unlocked.
 * EVA: Complete Tsavo Highway on Normal, Heroic, or Legendary difficulties.
 * Hayabusa: Collect at least 5 skulls.
 * Katana (Add-On) : Earn 1000 out of 1000 Gamerscore Points for Halo 3
 * EOD: Complete Tsavo Highway on Legendary difficulty.
 * Scout: Complete "Too Close to the Sun" Achievement.
 * Recon: Bungie Employees and a selected few players.
 * Flaming Helmet Chest Piece: Bungie Employees only.

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 Elite armor permutation combinations: one helmet, two shoulder pieces, and chest piece per permutation.

Helmets

 * Combat: Default Unlocked.
 * Assault: Default Unlocked.
 * Flight: Complete Campaign on Heroic or Legendary difficulties.
 * Ascetic: Unlock "Steppin' Razor" Achievement.
 * Commando: Unlock "Overkill" Achievement.

Shoulder Armor

 * Combat: Default Unlocked.
 * Assault: Default Unlocked.
 * Flight: Complete The Ark on legendary difficulty or "The Storm" on Legendary difficulty.
 * Ascetic: Unlock "Overkill" Achievement.
 * Commando: Unlocked automatically after Auto Update 1 was applied on February 19, 2008.

Body Armor

 * Combat: Default Unlocked.
 * Assault: Default Unlocked.
 * Flight: Complete Tsavo Highway on Heroic or Legendary difficulties.
 * Ascetic: Complete "Up Close and Personal" Achievement.
 * Commando: Complete "Triple Kill" Achievement.

Unlocking Permutations
Note that the Arbiter's armor cannot be unlocked, no matter how hard you may try.