Heat oven to 350°F (325°F for dark or nonstick pans). Grease bottoms only of 2 (8- or 9-inch) round cake pans. Make cake batter as directed on box.
Divide cake batter evenly between two separate bowls (about 2 cups each). In one bowl, add 25 drops yellow food color and 4 drops red food color; mix well. Stir in orange peel. Pour into one of the pans. In other bowl, add 25 drops neon green food color and 2 drops neon blue food color; mix well. Stir in lime peel. Pour into other pan.
Bake and cool as directed on box for 8- or 9-inch rounds.
Place green cake layer, top side down, on serving plate. Spread 1/3 cup frosting on top. Top with orange cake layer, top side up. Frost side and top of cake with remaining frosting. Sprinkle cereal on top of cake.