Grease and lightly flour one 9" round pan and one 13x9x2 pan; set aside. In a very large bowl stir together flour, sugar, baking powder, soda, and 1/4 teaspoon salt. Add milk, shortening, 1/2 cup champagne, vanilla, and, if desired, red food coloring. Beat on low to medium speed about 30 seconds or just till combined, scraping bowl constantly. Beat on medium speed 2 minutes. Add unbeaten egg whites; beat 2 minutes. (Batter may appear slightly curdled.) Spread 2-3/4 cups batter in 9" pan and remaining batter in 13x9x2 pan. Bake at 350~F 30-35 minutes for round cake and 40-45 minutes for rectangular cake or until cake tests done. Cool cakes in pans on racks 10 minutes. Remove from pans and cool completely. Using cardboard circle patterns of 4 and 6 inches, cut one 4-inch and one 6-inch cake layer from the 13x9x2 cake. If desired, drizzle 3 tablespoons of champagne over the top of each round cake layer. Place the 9-inch cake layer on a serving plate; frost sides and top. Lining up the backs of the layers, place the 6-inch cake on top of the 9-inch cake; frost sides and top. Repeat with remainging 4-inch cake. If desired, tint any remaining frosting; decorate with tinted frosting and flowers. Serves 24. CHAMPAGNE BUTTERCREAM FROSTING: In a bowl beat shortening and butter or margarine until combined. Beat in champagne or cream sherry. Beat in sifted powdered sugar and a few drops red food coloring. If necessary, beat in additional champagne or cream sherry to make frosting spreading consistency. Makes 3-3/4 cups.