Prepare frosting for crumb coating. Mix all ingredients together until smooth. Place blue cake on a clean work surface covered with wax paper strips or a cake board
, if desired. Add a thin coat of frosting to the blue cake, top off with purple cake. Repeat frosting layer and top off with white cake. Carefully coat entire top and sides of cake with a thin layer of frosting, sealing in any crumbs. Transfer to the refrigerator to harden for at least several hours or overnight.