I've never airbrushed outside But I have painted a lot of house trim and such in temps down to zero. As a general rule, surface and material temps are what's important. 50 degrees is about the bottom. Things like keeping your blanks and paints inside are smart moves. Heat drying, which most people do at all temps, is what keeps the paint warm when is at its most vulnerable... when is thin. As long as it dries before it freezes you should be alright.
Best of luck to you outdoor air brushers, I like to stay inside for those sub zero days. Been a nice winter so far!