IMBC leftovers should be refrigerated. I've read a few recipes that said IMBC can sit out for 24 hrs at room temp or 1 wk in the fridge. I'd be curious to see what others say. No clue about airbrushing, sorry.
I just googled the subject and here's what I found.
http://www.cakecentral.com/modules.php?name=Forums&file=viewtopic&printertopic=1&t=648025&start=0&postdays=0&postorder=asc&vote=viewresult
This is a response I found about airbrushing IMBC on google.
I use IMBC only and have gotten it to a medium color only - so you're right, it doesn't take to dark colors but does take well to the new ELECTRIC colors. Try those. They're more intense without adding a lot of liquid or gel to your icing.
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