To use any camo in Modern Warfare 3, you first need to unlock it. The Base Camos are unlocked by killing enemies using the weapon required in unique ways.
For example, the Scales camo asks you to kill 50 enemies with the M4, and the Desert Hybrid requires you to kill another 50 while using its scope.
These specific challenges need to be unlocked first by leveling up the weapon you’re going to use. Each gun has its specific level at which it gets its camo challenges unlocked.
You can check this and the camo themselves by going to the Weapons tab, selecting any Multiplayer Loadout, customizing it with any gun you want, and then going to the Blacksmith menu of that weapon.
Here you’ll first see all the attachments you can add to it. You need to move from the Build Weapon tab to the Customize one.
Choose the first option (Camo), and you’ll be able to check the gun’s specific camos on the left and all the ones you’ve already unlocked on the right.
To unlock the Gold Camo, you need to first unlock all four Base Camos the weapon you selected has. Once that’s done, you’ll unlock the challenge specific to the Gold Camo.
For example, the M4’s Gold Camo asks you to get three kills without dying ten times — killing the same player over and over counts as long as you don’t die.
You can level up the weapon you want and complete its challenges in basically every online mode available (this also includes the co-op missions). Killing both real players and bots counts as long as you get the kills in the way it’s asked in the challenge.
While you can play anything you want, leveling up your gun near to its max level and completing every challenge can take some time depending on your skill and luck.
One partial solution to this problem is to use Double Weapon XP Tokens, which will duplicate all the experience points you win in a certain amount of time. You can know more about these tokens and how to get them in our Double XP guide.
Another useful tip is to play online modes that don’t have objectives apart from defeating enemies. For instance, Free-For-All, Team Deathmatch, and Kill Confirmed. Here you’ll focus on just killing soldiers, which will both level up your gun and let you complete challenges.
The easiest multiplayer mode for unlocking any Gold Camo, however, is Invasion. In this mode, you fight against an army of both real-life players and enemy bots. The bots are usually easier to defeat, and they aren’t as dangerous to you. Play a couple of matches here and you’ll unlock the desired Gold Camo in the quickest way possible.