Prepare and bake cake according to package directions, using two greased 9-in. round baking pans. Cool for 10 minutes before removing from pans to wire racks. For frosting, in large bowl, cream shortening and sugar until light and fluffy. Beat in water, salt and vanilla until smooth. Place one cake on a serving plate; spread with 2/3 cup frosting. Top with remaining cake. In small bowl, combine 2/3 cup frosting and blue food coloring. In another bowl, combine 1-1/2 cups frosting and red food coloring. Fill pastry bag with 1/4 cup white frosting (or use plastic bag and cut a small hole in corner of the bag) and set aside. Frost cake top and sides with remaining white frosting. With blue frosting, frost a 3-in. section in the upper left corner of the cake. Pipe white stars over blue frosting. Fill another pastry or plastic bag with red frosting (as with blue, above). Pipe stripes across top of cake.
This cake makes a pretty presentation on a picnic table. It is made from a boxed cake mix, then topped with sweet homemade frosting. Serves 12-14.