I have had that issue in the past with pearled airbrush colors (AmeriColor). I read somewhere to thin it out with some alcohol (vodka) so it didn't clog the tip and to clean after using by running vodka through the airbrush. I did that with my Valentine cookies and it worked very well. I just put some vodka in a small squeeze bottle and kept it beside me. I would use approximately drop for drop - maybe two of vodka per 1 of color because the color drops were bigger I noticed. I also made sure I shook the heck out the bottle of color - the metallic seem to separate even just sitting there a few minutes. I put it on low and did two or three thin passes to get the level of color I was seeking. I don't have a lot of airbrush experience, but I do like the pearlized colors!
