Silicone is kinda hard to paint, as so many things just dont stick to it. For silicone the best solution is diluted silicone (Xylene dilutes it) with pigments added. Silicone bonds to itself for good so the paint will become a permanent part of the bust. If you were casting yourself, the perfect option would be to tint the silicone to get an initial hue and then paint over it. There are also alcohol activated make-up meant for silicone appliances, it gives you nice results though I have never used it in a permanent piece so whether it would stand the test of time, I don't know.
Whatever the medium, the basics are the same. Once you master the color technique for, lets say getting skin look realistic, you can apply that to different mediums easily.
As for equipment, many have personal preferences. I use an Iwata HP-C Plus airbrush which has served me really well in modeling that requires fine detail as well as prop building and also make-up. Whatever airbrush you use, the absolute key is cleaning. I really can't stress the importance of cleaning enough, it is vital. another thing linked directly to cleaning is using paints that are thinned enough, filtered even. Especially with fine detail airbrushes the tolerances are so tight that a tiny speck of dried paint inside the airbrush will affect your result. Airbrushes are a high maintenance thing! They're also slow to work with. Not only it takes time to clean them between colors (assuming you have one brush without changeable paint jars), you really need to work in very thin layers to get good results. It is very tempting to do a good coverage with an airbrush (been there) in one go. Overall, you just need to be aware of the characteristics of airbrush work, the dos and donts. It is a rewarding tool.
In airbrushing, one thing you really should invest in is a compressor. Preferably one with a tank, as tankless comps can have a pulsating airflow. With a tank you get a steady pressure which is kinda important. You won't get that with car tires or cans of compressed air. And you need to be able to adjust the pressure as different mediums and different types of work may need different airpressure. Moisture trap is also neccessary so you dont get any water from the air supply. Most comps I have seen have had the moisture trap in them. Some like to use an in-line moisture trap as well to make sure there really is no water in the airflow.
Whether its an airbrush or a regular one you'll be using, the most important thing is to practice. Paint as much as you can, you'll only get better by doing it. You'll learn a lot about paints and techniques that way. As for reading, look in model building magazines, maybe make-up FX books, go to modeling websites and search for articles and tutorials. Best of luck!
