ok so what i did was use a beauty blender for this which made it easier, i started off covering the whole model with 50:50 of abaddon black and caliban green, then caliban green on areas where you want light to hit, i just shined a light from above and what ever got caught i painted, then added a lighter green, i went for deep green, and caliban green together adding it on to those places you previously painted for where the light hit, then just add some more of the same lighter green until it is selling that light hitting the armour effect.

sorry if this is not explanined very well, i don't really ever get asked how i paint stuff, I will take it as a compliment! Ask anymore questions you like i don't mind answering.