When you are editing your weapon in gunsmith select the customise tab at the top then camo in the lower left, hover over each camo to see the stats, its the left 8 you are interested in, the first 4 are the base camos for the weapon, you complete these first to unlock the gold challenge for the weapon then you need a certain number of gold camos within the class to unlock platinum challenges, for example in ARs you need 8 at gold before you can do platinum, then you need 51 weapons at platinum before you can do polyatomic and finally you need 51 weapons at polyatomic to unlock orion.
I got platinum on the m13b today and think it looks gorgeous especially on the health hazard build.