HEAT oven to 350ºF. Spray three 8-inch square pans with no-stick cooking spray.
PREPARE cake mix according to package directions, using water, oil and eggs. Divide batter into thirds. Color each third with food coloring to make pale yellow, pale green and pale pink batters. Pour batter into prepared pans.
BAKE 20 to 25 minutes or until toothpick inserted in center comes out clean. Cool 10 minutes in pans on racks. Remove from pans. Cool completely on racks.
TRIM edges and cut each layer in half to make 6 rectangles. Spread 5 cake layers with jelly. Stack layers, alternating colors. Top with remaining section. Turn entire cake on its side so layers are vertical. Place on serving plate. Wrap and refrigerate while preparing frosting.
BEAT shortening, powdered sugar, vanilla and milk until creamy. Blend in food coloring to color frosting a pale green. Frost top and sides of cake. Refrigerate until serving. Cut thin slices of cake across layers to create ribbon effect.