If you want to unlock them through in-game challenges, you can join any public game and manually add bots through the console.
Alternatively, this tutorial shows how to unlock all the camos (without grinding): https://forum.plutonium.pw/topic/5/tutorial-how-to-get-diamond-camo-in-plutot6?_=1590475934277
As far as I'm aware, it's currently impossible to unlock camos through dedicated servers or through Custom Game.