Heat oven to 350°F. Spray bottom of 13x9-inch baking pan with no-stick cooking spray; set aside.
Combine sugar, 3/4 cup butter, eggs and vanilla in bowl; beat on medium speed until well mixed. Add flour, pumpkin pie spice, baking powder, cinnamon, baking soda and salt; beat at low speed until well mixed. Gradually add milk, beating well after each addition. Stir in sweet potato.
Pour batter into prepared pan. Bake 27-32 minutes or until toothpick inserted in center comes out clean. Cool completely.
Combine powdered sugar, cream cheese and 2 tablespoons butter in bowl. Beat on low speed until smooth and creamy. Place 1/2 cup frosting in bowl; stir in enough food color for desired tint. Spoon green frosting into resealable plastic food bag; snip off 1 small corner of bag.
Frost cake with remaining white frosting. Pipe green frosting over top of cake to resemble pumpkin vines. Roll each gumdrop with rolling pin to about 1 1/2-inches in diameter. Cut out leaves with small knife or canapé cutter. Place leaves and pumpkins randomly along green frosting vines on cake.
*Substitute 3/4 teaspoon ground cinnamon, 3/4 teaspoon ground ginger and 1/4 teaspoon each of ground nutmeg and cloves.