I use a similar all-butter icing recipe and I find that whipping in the tiniest drop of violet color will help negate some of the yellow cast to the icing. If the client wants the icing whiter, I have to use whitner, like Americolor’s bright white to make it super white. In this case, I use regular buttercream for the whole cake and crumb coat, and just use the super whitened version for the outside coat of the cake. It really doesn’t affect the flavor that much. Hope that helps!