Hi...You'd mix a large batch of white buttercream icing; stack/fill your cake layers and crumb coat the whole thing with the icing. Put in the fridge to get hard. Keep 2-3 cups of it white for the contrast piping and divide the rest in half; make half pink, the other half blue. when your cake is cool, mark how you want the cake divided with the point of a knife or a skewer. Using the line as a guide, frost half the cake with pink and half with blue. Make it as smooth as you can. Then using the white frosting, go over the where the 2 colors meet with the flat tip and then change tips and pipe the border. Hope this helps.
that division is pretty easy to get to with a spatula like tootsie says/recommends -- sometimes I pipe the icing at the join so I can control it easier and pat it down with a smooth towel -- just another option
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