Painting on MMF?
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Could you describe what you are painting? Is this during the painting process? Or does this happen after the paint has been left to dry?
It sounds like it could be your paint. What are you using? Like lolarx8 suggested, food coloring gels + alcohol work well. Sometimes I paint with airbrush colors (w/o the airbrush, you know?) straight from the container. You want a color agent plus something that evaporates quickly like clear alcohol (e.g. vodka, Everclear).