Urban's Colour Reference Charts
Some modellers prefer enamel paints, other acrylics; both types of paint have strong supporters. Which type of paint you use is a matter of personal choice. Enamel paints generally provide a tougher finish and a higher gloss, while acrylic paints dry much faster and are often water-soluble, which makes them easier to clean up.
All model paints, regardless of type, must be thinned for airbrushing.
The amount of thinner depends on many factors. These include the brand of paint, the temperature, the air pressure being used, the width of the desired spray and the required thickness of the paint coat. Most manufacturers will recommend a thinning ratio on the paint jar, but the best recipe for success is experimentation and practice. The typical ratio range of thinner to paint is probably between 20/80 and 50/50. Higher proportion of thinner may be required for fine-line work.
Thinning paints is pretty straightforward - all manufacturers offer appropriate thinners, or modellers may choose to use a generic brand like Mineral Turpentine for enamels, and Isopropyl Alcohol or just Distilled Water for acrylics.
Of note is that some acrylic paints are more temperamental. Aeromaster, Floquil, Polly "S" and Pactra acrylics may only be thinned with distilled water or Polly "S" thinners. Even then, when used for fine-line spraying, these paints will tend to clog in the airbrush nozzle. One way to correct this problem is to add a few drops of Acrylic Retarder Medium to the thinned paint mix. This will slow the drying process enough to allow very fine work. Acrylic Retarder is available under many different brands from art supply shops.