A camo is basically a weapon skin that lets you customize your gun’s appearance, along with other cosmetic items like charms and stickers.
This is only a visual change — it won’t give you any boost or advantage while playing the game.
Each weapon in Modern Warfare 3 has four Base Camos and four Completionist Camos.
The Base Camos are four unique skins that can only be unlocked with one weapon and then used with any gun you want, while the completionist ones are always the same styles but exclusive to the weapon you use.
For example, the M4’s first Base Camo is Scales. If you unlock it, you can use it on your M4 or in another assault rifle, shotgun, etc. However, if you unlock an M4 Completionist Camo, you can only use it with this gun.
Meanwhile, the Gold Camo is the first completionist camo that you can unlock with any weapon. As its name implies, it will add a golden visual to your gun, making it shine on the battlefield.
Before being able to use it, however, you’ll have to complete certain challenges.