Unlockable Cheats
The following cheats can be unlocked by obtaining enemy intel, which you'll find throughout the Campaign Mode.

CoD Noir: Turns all gameplay black and white, giving the game a classic war movie feel. Collect 2 pieces of enemy intel.
Photo-Negative: Inverses all of the colors of the game. Collect 4 pieces of enemy intel.
Super Contrast: Dramatically increases the game's contrast, making the darks much darker and the lights much lighter. Collect 6 pieces of enemy intel.
Ragtime Warfare: Gameplay goes black-and-white, dust and scratches fill the screen, it plays at 2x speed, and the music becomes piano music. Collect 8 pieces of enemy intel.
Cluster Bombs: After one of your frag grenades explodes, four more explode in a cross-shaped pattern. Collect 10 pieces of enemy intel.
A Bad Year: When you kill enemies, they explode into a bunch of old tires. Collect 15 pieces of enemy intel.
Slo-Mo Ability: By using the melee button, you can change the game to slow-mo and play at half-speed. Collect 20 pieces of enemy intel.