If you're talking about Parma Faskolor, there's no need to thin it. If you still want to thin it, maybe it's clogging the airbrush, you can using about one part water or airbrush media (or reducer, different companies, different names). Spray Faskolor around 40 psi and you should get good results.
Also, there is a Painting forum on RCTech and this question as well as many other "painting" type questions are answered there. Check it out, the guys and girls there are very helpful.
Good Luck