Once you unlock all of the camos for a single weapon, you unlock Golden Viper. You then need to unlock Golden Viper camo for every weapon in a class (all assault rifles, all submachine guns, etc.) to unlock Plague Diamond.
How do you get plague diamonds in Cold War zombies?
Plague Diamond Camouflage is a mastery camouflage in Call of Duty: Black Ops Cold War. It is the Zombies variation of Diamond Camouflage, and is obtained by unlocking Golden Viper Camouflage for each weapon in its respective class.
How do you get the diamond in zombies?
Unlock diamond camo for all weapon classes. The trickiest camo challenge appears to be in the ‘Infection’ category, which requires us to score 20 or more consecutive kills with our weapon without taking damage.