For filling, beat shortening and 6 tablespoons butter in large bowl with electric mixer until light and fluffy. Gradually add sugar, one cup at a time, beating well on medium speed. Scrape sides and bottom of bowl often. When all sugar has been mixed in, icing will appear dry. Add 1/2 teaspoon of each citrus zests, juice and, if desired, liqueur; beat at medium speed until light and fluffy.