Heat oven to 350°F (325°F for dark or nonstick pan). Spray bottom only of 13x9-inch pan with baking spray with flour.
Make cake batter as directed on box, adding 1 1/2 teaspoons of the mint extract with the water. Reserve 1 cup batter. Stir 3 drops of the green food color into reserved batter; set aside. Pour remaining batter into pan.
Drop green batter by generous tablespoonfuls 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 one-fourth turn; repeat cutting for swirled design.
Bake as directed on box for 13x9-inch pan. Run knife around sides of pan to loosen cake. Cool completely, about 1 hour.
Carefully spread fudge topping evenly over cake. In medium bowl, stir whipped topping, remaining 1/2 teaspoon mint extract, remaining 9 drops green food color and the yellow food color until blended. Spread whipped topping mixture evenly over fudge. Garnish with candy pieces. Store covered in refrigerator.