b&q tester pots, and for larger areas, for a couple of quid, they will supply a small pot of any colour you want, just take the paint swatch to the mixing counter, or take a sample of what you want and they scan it. Thin with screenwash or window cleaner, I use screenwash, and use a clear matt, satin or gloss clear laquer, depending on the finish you want. I spray at 30 to 40psi, and clean the airbrush a lot!
It also brushes on nice, depending on the surface. Im more planes now, but used same method on some of the boats, here are examples, the catalina was brush painted, with an airbrush to get the feathering effect, the zero was airbrushed, and the YAK jet was airbrushed, all using these paints