I stack a cake, and airbrush all 3 tiers the same color, to have the white part show underneath when they remove the tiers to cut?

Thanks so much.

Not really... Depends on who the cake is for and how much work you want to put in to it... For the Queen of England airbrush each tier, a neice don't :0)
However if it's one of those things that will haunt you after the cake goes out the door then I say airbrush each tier 1st then assemble..