Whites aren't white enough. It seems like the white is mixing in with the background paint. P.S. Excuse my lame artwork, I've only been airbrushing for about a week now. Nothing lame there.
Whites are always a pain.
Look into the different types of airbrush paint available. The best I've found for defeating this problem is Createx. It's still not as opaque as I would like it to be on dark fabrics, but it's better than any other I've tried. It seems like the white is mixing in with the background paint. Nothing lame there.