Hey Adler doing the splotches can be a real pain, I do mine freehand I usually get them pretty good first time but not always. When I do stuff it up its usually the last colour, which means starting again with previous colours covering what I'm not happy with and carefully reapplying the layers.
I use Tamiya acrylics, fine nozzle in the airbrush, thin the paint 50/50 to start with, spray onto pad paper at varying distances to create the splotches/mottle pattern. Differing pressures, vary the mix and practice.
What scale are you working with? Remember the smaller the scale the more difficult it is to do.