Scratch that last comment about Zero being three out of the four skins, as Gearbox has decided to include Gaige the Mechromancer, making the total number of skins five. These codes will, like the other Halloween ones, will last until Sunday afternoon. This code set will unlock the Stinger Missile skin for Gaige the Mechromancer.
PC: W3WBJ-C6ZB5-C6JJT-33JJB-JS
Xbox 360: WTKTJ-696S5-R9RB6-5RTT3-BT
PlayStation 3: CJWTB-FZK5T-TKKWT-ZJ5B3-X5