While I can't recommend
GW colours, I would recommend our Sandstone brown, Mud brown and Dirt or Dust brown.
Easiest way to get an idea for Camouflage is just by looking for patterns and then replacing the colours with your own so Black areas would be Sandstone Brown (Dark Brown) etc., getting it right isn't easy, this is our ethereal Tau with an urban camo scheme, we had to take the first pattern off and redo it to make it look more natural.
First attempt
Second (Final) Attempt
In your case, because it's armour rather than a cloak, you don't have to worry about shading first, you can apply the pattern first and then apply the shading wash afterwards.
However if you don't wish to go for a pattern, then just the colours or any kind of progressive browns in largish steps would work perfectly fine.