Here is a tip i picked up for thinning Tamiya acrlics and i have had no problems so far, its to use windshield washer fluid for thinning acrylic paints. Now, every time I airbrush, I get great results. The WW fluid (I use the cheap stuff from a gallon jug, sold at discount stores all over) has some detergent in it, which helps to break the surface tension of the water-based paint, allowing it to flow more freely. I use the blue tinted stuff, but the color has no impact on the color of the paint- even when painting with white or light yellow.

Hope this helps

Ian