Dave,
The process of "shading"--whether PRE- or POST- shading is the process by which you attempt to simulate the way that light works upon and plays over an object. It involves adding highlights/lowlights, via the airbrush.
I use a somewhat "extreme' method, where I use several shades of altered colors to make a sharp distinction between hues. I mostly do this on "green" tanks, but can also do it on German armor, albeit to a lesser degree.
Here is what my ersatz M10 Panther looked like after I shaded it:
There are three or four shades od OD sprayed here. One dark base, to which Black has been added. Then a straight OD layer Then two progressively lighter coats, one lightened with Dark Yellow or Buff, and one with just straight Dark Yellow sprayed very very lightly for tinting purposes. The lighter colors are sprayed into the centers of the panels, leaving the dark to "outline" the lines of themodel, and leaving "shadow" in the creases.
It looks a bit cartoonish, does it not? However, with washes, drybrushing, filters, pigments, etc-all thetricks of weathering--it tones down into a nice finish:
I hope this gives you an idea of the technique It's really not hard; it just takes a bit of courage and pluck. Soe guys don't like the result or the technique, but that's ok. I just posted it here to give you another point of view and possible option in your future builds! 