Agreed, there are some excellent brands of Airbrush color which are easily accessible and go a very long way (you only use a few drops at a time.) Gel colors are just too thick and not designed for the fine head of an airbrush and you don't want to be clogging that baby -- they cost almost as much as the entire unit to replace! 
Just something to keep in mind. 
You CAN use just regular old food colors~ I get them by the quart at restaurant supply or places like Price Club. The KEY is in the cleaning of the airbrush! Even the 'airbrush' colors will eventually clog the airbrush if it is not thoroughly cleaned after use!
To me the main advantage of the airbrush color is the amazing choices of colors available! Yes, you CAN mix your own colors, and using the 'regular' kind for pure primary colors is easy, but remembering how many drops to mix a special color is not easy to do when you get busy! So I use both kinds! and I clean the airbrush by back flushing with warm water [ just hold your finger over the tip with the compressor running. For a 'deep cleaning' now and then I use a solution made with a disolved denture tablet!
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