I am kind of on the same wavelength. I have seen the most beautiful buttercream damask patterns on top of buttercream, but I couldn't imagine how to do it! Frozen cake? Thanks for posing the question!
i think not 100% sure) you ice your cake and let it dry/crust whatever you call it..lol the trace out your"pattern" on wax paper and entle line over the part of your cake you want to leave white and then airbrush the color on, then if you want to do another color cover the part you airbrush first, i am going to have to experiment with this, they may make templates for this kind of work too though!...hmm my guess is airbrush!
It may not be "the way" to do it, but this is what I'd do:
Ice the cake in white and smooth as usual
Freehand a wave pattern with a toothpick
Pipe the blue using a tip #7 (or 10 or whatever)
Let it crust and go over it with a paint roller (Melvira method)
Pipe the border
I only use a crusting buttercream. The above method would not work with a non-crusting one.
Sounds pretty easy in my head. 
Hi,
I've done several cakes in two tone buttercream. I use the big icer tip and pipe the bottom color first. Then change colors and pipe the other color on top. Then smooth the line with a spatula, paper towel and or roller. Works like a charm!
If the pattern isn't just straight lines around the cake, just angle the icer tip to the desired pattern like a wave and do the same as above.
KimAZ
Eerrr! I've post this question an replyed to it and still didn't any notice that it had replies!! 
Anyway, I used to work with Patty so I know she doesn't do it with an airbrush. We didn't have one till right before I left there.
I know one time she free-hand painted it one, I think with lucter dust. But this cake, doesn't look luster or shiney like it was done that way.
Anyway, thanks for the suggestions. I think I might try experimenting with this next week. I'll let everyone know how it goes.
Thanks 
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