Line an 8-cup mold or mixing bowl completely with aluminum foil. Working quickly, spread the vanilla ice cream 1 inch thick over the bottom and sides of the bowl. (It'll probably slide down the sides a bit.) Immediately place in the freezer and freeze for about 1 hour. When the ice cream has become somewhat hard, but not frozen solid, spread it all the way up the sides to the top of the bowl; then replace it in the freezer for about 1 hour, until firm. Place the raspberry sherbet in a medium-sized bowl and add the chocolate chips; blend until evenly mixed. Place in the vanilla ice cream-lined bowl. Cover with plastic wrap and freeze overnight, or until completely hard. When ready to serve, in a small bowl, mix the green food color with the whipped topping until everly blended. Remove the mold from the freezer, remove the plastic wrap, and invert onto a platter larger than the mold. Remove the mold and peel off the foil. Spread the whipped topping evenly over the ice cream and serve immediately, or freeze until the topping is firm, then cover and keep frozen until ready to serve. Cut into wedges, just like fresh watermelon.

10 to 12 servings