Heat oven to 350. Lightly spray a 9" x 13" baking pan w/ cooking spray. In bowl make cake as directed. Divide batter into 3 bowl. Color 1 bowl w/ yellow food coloring.
Color 1 bowl w/ orange food coloring, leave 3rd bowl plain. Spread yellow batter in pan, put in freezer for 20 mins. Carefully pour orange layer over yellow layer, freeze 20 mins. Spread plain white layer over orange layer, let sit 30 mins.
Then bake as directed on cake box. Take out of oven and poke holes all over cake w/ the end of a wooden spoon. Pour sweetened milk into holes. refrigerate overnight. Top w/ cool whip. Decorate w/ candy corn.