Sounds like you are rediscovering a painting technique of pre-shading.
This can be done in many ways.
1) Spray with flat black to cover well. Then spray with flat white from overhead only. The spray "rains" down just like sun light highlighting areas that would be light.
2) Spray with flat black to cover well. Brush flat white on from head toward feet highlighting the upper most portion of the cast details.
3) Spray with flat white to cover well. Wash with flat black and allow to pool in recesses.
Now paint with transparent (well thinned) paint to allow shadows to be seen through the paint.
If you are painting over the effect, I would say to avoid the expense of actual Army Painter wash. Using a Min-Wax Tutor stain will be more cost effective. But making up a wash of black would be quicker (Min-Wax and Army Painter have long drying times before one should paint over them).
I have painted over them in the past, but you have to wait a long time to make sure all the solvent it out of the varnish.
You did not say what paint (media) you use for your painting. But assuming acrylic, Future Floor Finish, is an extremely low cost and effective wash media to add the color of your desire to. You can prime white, wash with black, and then paint with no extra waiting time.
Here are my very old pages on "Dipping" techniques:
link
link
But more importantly the classic work: "The Complete Future": link
I see it has been up for 12 years now! It details some of the many modeling uses of Future Floor Finish (now sold under other names).