I am just finishing off a spiderman cake, 2 tiers buttercream, airbrushed bottom tier americolor dark blue, laid it in the fridge, then airbrushed top tier red, put 2 tiers together and laid back in fridge while I finished off some fondant accents to go on the cake. When I took the cake out of the fridge a little later I noticed my bottom airbrushed blue tier is now purple, not sure why this happened, the blue worked perfectly a couple weeks ago on a fondant toopy and binoo cake I made. The red is fine. Does some airbrush colors work differently on buttercream cakes?

Is it possible that some of the red wasn't dry & some how seeped into the blue after you put the two together (were both tiers dry when you put them together)? those two colors together makes purple..that's the only thing I can think of.

Ahhhhhh, I say that is what happened, I put the 2 tiers together before they were dry, thanks so much for your reply, every cake I do I learn something new

Just saw the picture... I don't think the red ran into the blue. It looks more like the blue was applied too heavy and ran. It left a purplish color where it's thinner and whatever red is in the coloring stained the buttercream more. (I know I probably didn't describe that very well... hope you know what I'm talking about... lol). When you're airbrushing color that deep, you should do it in several light coats, making sure to let it dry COMPLETELY between each coat.