Hello eshaw,
We actually use a variety of paints depending on the application of techniques. Most color filling is done to areas that only receive one color and allow the masking off of other areas that need to be a different color. For 80% of etched items, such as bottles, cups, drinking glasses and awards, we use spray paints of good quality, such as Krylon or Rustoleum. You need to follow the instructions for each product as indicated on the paint can, since all of them have different drying and curing times. Always make sure that paints are properly cured before turning an item over to a customer.
For some art and decorative items, we have used Delta paints, which can be obtained through craft and art supply stores; these paints offer a variety of pearlized and metallic options that spray paints do not, and these paints can be easily hand/brush painted. For elaborate paintings that require gradations and modeling, we actually use an airbrush with water-based airbrush paints that dry permanently after curing. This, of course, requires the acquisition of skills in airbrushing techniques in order to be successful. There are many different brands of airbrush paints on the market, and you will have to match the brand up with the type of airbrush you would be using.
I hope this helps you a bit and/or at least gives you more to think about. As with all techniques, practice is required, and it is a very good idea to seek instruction in these techniques, which will save you considerable time and frustration, and, of course, actually money as well. Let me know if you have further questions.