Your first order of business in shading is to have the figure sprayed in its final colors and all details applied and finished. A shade effect is how to exaggerate lighting in the room, so it will be the final stage of any spray.

These are the parts I'll be using. They are complete in detail and color application:




Using just ordinary masking tape again, I create a frame in which I want to shade. This keeps any excess paint off of my higher areas which would be the most likely place a light source would illuminate:




Seen here, I used the airbrush to shade heavier on the top of the panel where light would be less likely to get to. The fake light source is implied to be to the figure’s right side and higher up. The lower section was shaded to suggest the whole panel is sunken and rolled. The inner edge gets no paint as it's completely exposed to the "made-up" light source:




Expanding on the light source bit, we can highlight panel lines to accommodate this illusion:




Begin by deciding where this light source will be coming from (try to keep it somewhat consistent throughout the figure). Now, that does not mean that because the light source is up and to the right of the figure that the entire left side needs to be black - it just means to try not to put your shaded areas in direct opposition.
The first step here would be to lightly spray out the lower edge of this line:




If there are any 90 degree angles, they will need to be filled in after the first line has dried:



. . . . and the final product looks like this:


If you so choose to go further and stay true to the light source theme, you can lightly shade the entire inner leg. The shading you've already done will darken along with it. You can stay quite light with this technique or even apply it dark so long as you stay relatively consistent you should be ok.



Here's a side by side comparison:




Here's a shaded face panel:




Creating grid work can give the illusion of a busy project without having to do a whole lot of etching or creating separate panels. To do this we . . . start by spraying out our base color which can be anything but black:




Using a strip of masking tape, gently apply it to the area (usually someone wouldn't use masking tape as the crepe in it causes fuzzy spots but because we'll be spraying in such light and dry coats, product or tape pressure application is inconsequential). Using your airbrush, lightly spray along the two longest sides of the tape:




With the tape pulled away, it will look like this:




Using a new piece of tape, gently apply it to the opposite side of one of the original leading edges:




Using the airbrush, apply your black paint lightly to the new surface. The reasons we stitch it together like this rather than freehand are:

1. [obtaining] a foolproof straight line;
2. the point in which both shaded areas intersect will appear darker and give the illusion of being an actual line;
3. the natural shaded area appears three dimensional by creating depth:
   
   
   
   
   We now start our horizontal lines in the same fashion:
   
   
   
   
   Again, highlight the opposite ends of the initial spray:
   
   
   
   
   . . . . and move around the whole project with this technique to fill in areas that may appear bland otherwise:
   
   
   
   
   A side by side comparison to the difference this technique can make: