I have seen a bunch of buttercream covered cakes that have different colored sides than the top. How do you do this and keep the line clean when you are smoothing?
Well, one way is to frost the cake in white and then air brush the two colors.
I covered the top first, then the sides...used a border to cover (hide) the edges where the two colors meet. I did this with the wedding cake in my photos. Hope this helps.