Rainbow Citrus Cake

Preheat oven to 350 degrees F (175 degrees C). Grease and flour three 9-inch pans. Sift together the flour, baking powder, and salt. Set aside. In a large bowl, cream together the shortening and sugar until light and fluffy. Beat in the eggs one at a time, mixing until each egg is incorporated; stir in the vanilla. Beat in the flour mixture alternately with the milk. Divide batter into 3 bowls. In the first bowl, stir in lemon zest and yellow food coloring; pour into prepared pan. In the second bowl, stir in orange zest and orange food coloring; pour into second prepared pan. In the last bowl, stir in the lime zest and green food coloring; pour into third prepared pan. Bake in the preheated oven until a toothpick inserted into the center of each cake layer comes out clean, about 30 minutes. Let cool in pan for 5 minutes, then turn out onto a wire rack and cool completely. Assemble the cake: stack the layers together with the Lemon Filling in between the layers. Frost sides and top with Orange Cream Frosting. Refrigerate until serving.