Preheat oven to 350°F. Spray 9-inch round cake pan with vegetable pan spray. Prepare cake mix with pureed watermelon, oil and egg whites. Gently stir in icing color to desired color; add mini chips. Pour into prepared pan. Bake 35-40 minutes or until toothpick inserted in center comes out clean. Cool in pan 10 minutes. Remove from pan and cool completely.
Remove crown from cake. Place on foil-wrapped cake circle. Ice outer edge of layer about ¼-in. thick with white buttercream. Color fondant light green; roll out 1/8-in. thick. Cut a strip 28 in. long x the width of the cake layer; attach to cake layer over buttercream. Add stripes using green FoodWriters.