In my experience, this process is better thought about beforehand; you could paint it in a lighter color and then use an overall darker green wash to darken the recesses and shadow area; the wash will do that very effectively. Small scale pieces are hard to get to look realistic since there is so little surface area for the light to bounce off of--so you have to learn to simulate that effect with paint and painting effects. This is called "Scale Effect".
If you've already painted it a dark Dark Green, I would suggest lightening e base color with a bit of Tamiya Buff or a tan color and spraying the insides of the larger panels on the top parts of the tank. Or, if you feel adventurous, you can try using oils to lighten up the upper parts of the tank where light would hit it. Next, add a very thin wash of green+Black to instill shadows.
The most honest thing to say is that there is no "pat" recipe for this kind of modeling. You have to experiment a bit and ask yourself what you're trying to achieve and then work slowly, re-evaluating your work as you go.
A good trick on small, darkish armor is to lightly dry rush with tan.This will bring out the lines and panels. You can then further lighten certain key elements once you can better see their definition by using oil paints of greens and white or tan.