An airbrush is great for a lot of different things. Base coats are just the beginning of a long laundry list of different uses that you can put this fantastic tool to effective use. One such technique I've recently began to use with a variety of models I've worked on has been zenithal highlights.
Zenithal highlights break down to omni-directional light sourcing. It's less intense than Object Sourced Lighting, but gives a nice natural feel to your highlights without much sacrifice in time when you've gained an understanding of the overall intent.
When starting off you need to pick your lighting direction. Find a dynamic angle that works for the piece. In this case, I've gone with a slightly in front of the model spot and about 6 inches up and off the piece. The model was primed with Krylon white primer, and I then used my airbrush to go in and add the shading from the underside with VMC Black. This pre-shading changes the hue slightly of our base colors and will help down the road some in respect to washes and depth. You can also create a very nice mood depending on the amount of drama you're setting for the 'scene'. You could likely get much the same effect as I have here using black and white spray primers with a bit more of a controlled spray and careful attention to your angles.
Going back in with my airbrush I've sprayed the base color red, in this case VMC Carmine Red, to the areas I'm working with. This was applied to all of the red areas with no adherence to a certain angle, just getting the base layer down in quick fashion by using my airbrush. The real benefit to using an airbrush over hand brushing your base layer on is the speed and smoothness. Colors that would have taken an hour to paint by hand and get smooth, take minutes.
Going back to my lighting angle, I airbrushed on my first highlight, VMC Flat Red. Maintaining my zenithal angle for this step is crucial to getting the look right. You might feel inclined to dig in and start highlighting everything on the piece, but you'd quickly ruin the effect. Stick to the angle and you'll be able to brush in variations in hue and wash other areas down later.
After your airbrushed highlight, you can go back in and settle other areas with their base colors. For my Angels Sanguine, this means making the color scheme split with VMC German Grey.
As I said above, you can begin to create more variation in the piece with washes and paint by going into recessed areas and toning the colors down as they need it. I used Thraka Green and Gryphonne Sepia to get the higher armor plates to stand out more. Careful application is the name of the game here, don't go overboard and be sure to water them down some as you're using them. Highlights were added sparingly along armor edges and joins and were also kept low key to allow the lighting effect to work. The black areas were washed down as well with Badab Black carefully applied to create different tones in the armor. It was then brush highlighted to mimic the lighting effects.
Overall this is a really useful technique that many out there can make good use of. Don't have an airbrush? Try careful use of spray cans instead. The idea is also possible using regular bristles, but will take longer to accomplish. If you have a one off piece or are just looking for a new way to add some mood to your latest figures, I highly suggest you give this one a try.
Have some insight that I might have missed? Share your experiences with this technique in the comments! Knowledge is power!