I've painted 3 layers of gold luster on royal icing (see Teacup in my pics), never used the airbrush on it though. Test patch would be good. It depends a lot on how you spray...saturation is not what you are shooting for. Spray a light coat, let dry, repeat...that would be my suggestion.
I have sprayed the luster sprays on dried royal icing before. I did some cookies for a wedding shower and once the royal icing was dry I used a pink luster spray to do the monogram, and then i went over it again with a pearl luster spray. I used the luck's brand for my airbrush
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