Preheat oven to 325 degree F. For shortbread, in a large mixing bowl beat butter and sugar until combined. Gradually beat in flour and cornmeal until combined. Stir in nuts. Divide dough in half. On two large ungreased cookie sheets, press each half of dough into an 8-inch circle. Prick each circle several times with the tines of a fork. Bake about 20 minutes or until edges are lightly browned. Cool on cookie sheets 10 minutes. Remove each round to a wire rack. Cool completely. Whip cream to stiff peaks. In a medium bowl stir together milk, 1 teaspoon of each peel, lemon and lime juices, and food coloring. Fold in 2 cups of the cream. Cover and chill until ready to assemble. Cover and chill remaining cream.
To assemble, place one shortbread round on a serving platter. Spread with half of the lemon curd. Top with half of the whipped cream mixture. Place second shortbread round on top of whipped cream mixture. Spread with remaining lemon curd and then remaining whipped cream mixture. Spread remaining 1 cup whipped cream on top. Sprinkle with remaining peels. Cover and refrigerate several hours before serving. Makes 12 servings.