I got an airbrush last Christmas and I've been trying to practice.  Which is better for airbrushing - fondant or buttercream?  I decorated a BC cake yesterday in deep red that was slightly frozen, and I didn't color it in layers (too impatient), and the whole thing was wet and the colors ran down the side.

I airbrushed a white fondant cake last week that in black and hot pink.  It seemed to take the color well and dried quickly but then later the fondant seemed to stretch on one side and had white cracks that I couldn't cover later to prevent blotches. 

One more question:  you know how coloring black or red buttercream from white can make a bitter taste.  How does airbrushing a white fondant cake black taste compared to just using black Satin Ice fondant?

I'm a real novice, too, with my airbrush but I think it is wonderful fun.  Like you I get impatient but I am learning that if you want a deep color then multiple layers works much better at it appearing even, without splatter or drips.  I found both buttercream and fondant took the color well but I do prefer working on fondant as I worry that the buttercream will become too wet and weep.

A trick I was taught to get black buttercream is to start with Chocolate Butter Cream, then you dont have to add as much food coloring. The other trick with black or red its to add the coloring then let it just sit for a bit and the color will get richer. Then you can reach the color you want without over adding the coloring. 

 It is much easier to use the black satin ice fondant rather than airbrushing white fondant to black, when your airbrushing it you wil need to go over the same area over and over to reach a nice black shade but by then it could start beading up and will only continue to get worse in the fridge. But! if you start with a darker shade of fondant rather them white then you wouldn't have to spray as much. You would still have the risk of it moving and cracking. When I have that problem its because I had put too much butter cream under the fondant.

Good Luck!