You can unlock differents skins in Borderland 2.. for this enter one of the following shift codes. You can enter them in Main-menu/Extras/Shift Code/ Now sigh up and type one of the codes.

For Commando's "Yellow Jacket" Halloween skin use
Shift code: KBKB3-J3R5J-BKCKT-H3W33-9F56C

For Mechromancer's "Stinger Missile" Halloween skin use
Shift code: CJ5T3-5H5KT-3KCKT-9JKBB-3SKHT

For Salvador the Gunzerker's "Killer Bee" Halloween skin use
Shift code: C3W3B-5JX5J-4C55J-ZJWTJ-RH3HR

For Siren's "Fright of the Bumblebee" Halloween skin use
Shift code: KTCTJ-ZHCKB-BKKW3-STCJT-3FZTW