For German buttercream you first cook a custard, which is then blended with softened butter. The pink buttercream is naturally coloured wiht raspberries or strawberries, and the light yellow buttercream gets its colour from orange juice. I use these buttercreams to fill and decorate my rose cake.
Use this buttercream along with my sponge cake recipe.
Note that potato starch is not the same as potato flour, though sometimes potato starch is erroneously labelled as potato flour. Make sure what you buy is a refined, white powder, as true potato flour is made from the entire potato, including the skin, and is less refined.