I typically use an airbrush as a beginning stage to lay down my foundation color on large areas of the miniature. Join me and I'll take you through my 5-step process that gives me a nice base-color with a little shading and highlighting as a basis for the rest of my shading and highlighting work.
Step 1 - Choose your colors.
You need your base color, a shading color, and a highlight.
In this case, my base color is the now out of production (OOP) Hawk Turquoise from Games Workshop.
To shade it, I chose OOP Dark Flesh from Games Workshop.
Side note: lately I haven't been using black to shade my models, but instead choose a color opposite of my base on the color wheel for shading. More on that at another time.
The third paint I chose was Vallejo Model Color Pale Blue, which is a really nice Turquoise with a little more green and white in it than Hawk Turquoise - perfect for highlighting.
Step 2 - Base Coat.
Mix your base color with some thinner (whatever you use - I use a mix of water and isopropanol) in a cup and load your airbrush. Apply a decent coat over the entire model as I have done below.
Step 3 - Shade
Mix your shade color into your base color and add some thinner. Don't over do it, you don't want your transitions to be too stark. In my case, I added OOP Dark Flesh to my OOP Hawk Turquoise in about a 1:2 ratio.
This is where the fun begins. Apply this color only in the lower parts of the model and the areas that would actually be shaded. If it helps, try to spray from an angle underneath the model, shooting upwards. This is why I have attached my models to these old paint pots as I can handle the models well spray and get different angles that I need.
Stage 4 - Highlight
Pull out your highlight color. In this case, I didn't mix my highlight into my base color, since they are already very similar hues of color, but different values.
With the highlight, I hold the models at an angle and spray downwards onto them. I do this from varying degrees until I am happy with how it looks. At this stage, the contrasts will be a little too strong so we are going to tone it down a bit.
Stage 5- Tie it together
As a final step, I mix my base color with a lot of thinner. I want a translucent coat to go down, that won't remove all the work I've just done.
Now what I do is spray the model all around at a 90 degree angle, hitting the models mid-section and lower. I leave the brightest highlights alone at the top of the model. See how it looks below.
And that is how I get a nice foundation of my base color, shaded and highlighted before I break my traditional brush and go to work. I'll still do a fair amount of shading and highlighting - especially some hard-edge highlighting, but at least this gets me 75% of the way there.
I hope you've found this useful and inspiring. Get out your airbrush and see what you can do!