There are many brands of paints made for airbrushing (some are oil based and others are water based). Some are already thinned to the righ consisntesy.

Water colors can also be used with good results on paper (but they are not permanet, they are not waterproof).

Go to an art store. Using the wrong paint can clog your airbrush.

If you use "regular" acrylic paint DON NOT USE CADMIUM PAINTS, they clearly state "NOT FOR AIRBRUSH" since they are very toxic if they are sprayed. And don't use the "heavy body", it's too think, better try the "fluid".

But again better try the air brush paints. That's what's they were made for.