Using the airbrush blocking techniques is a great way to speed along an airbrush project. This video is a superb example of it... much better than my example.
Basically, as you will see in the video, blocking is when you lay down the base colors of a painting with a paint brush and then tweak the color later with your airbrush to give it dimension.
Watch the video here first and I'll blather on a bit more below it:
This technique is especially useful when you are doing LARGE projects that are not on metal surfaces (metal surfaces will show brush marks) - specifically I am thinking murals of canvas work.
The easiest way to apply the technique is to start with a solid layer of the lightest color in that area. For example, if I were going to paint the front fender of the bike in the video, I would paint the whole fender area in the lightest shade of blue that shows up in the fender portion of the reference photo and then, when it was dry, I would come back with transparent paints in blues and blacks to add the dimension leaving the lightest areas light.
In the video, Steve Nunez (the artist) chooses a more midtone level of blue for the fender - it is not the lightest color and it is not the darkest. This works too, but, to make the very lightest area in the center of the fender he would have had to add a white to lighten it up. I personnally am terrified of white - it can go really wrong really fast - so I avoid applying white over darks (what a chicken I am!)