Heat oven to 350° for shiny metal of glass pan. Spray bottom of 13X9 inch pan with baking spray.
Make cake mix as directed on box, using water, oil, 1 ½ tsp of mint extract and the egg whites. Reserve one cup of batter. Stir 3 drops of the green food color into reserved batter; set aside. Pour remaining batter into pan.
Drop green batter by generous tablespoons randomly in 12 to 14 mounds onto batter in pan. Cut through batters with metal spatula or knife in S-shaped curves in one continuous motion. Turn pan ¼ turn; repeat cutting for swirled design.
Bake 28 to 33 minutes or until toothpick inserted in center comes out clean. Run knife around side of pan to loosen cake. Cool completely, about 1 hour.
Carefully spread fudge topping evenly over cake. In medium bowl, stir whipped topping, remaining ½ tsp extract, remaining 9 drops green food color and the yellow food color until blended. Spread whipped topping mixture evenly over the fudge. Garnish with candy pieces. Store covered in the refrigerator.