Preheat oven to 325F. Separate egg whites and yolks. Set whites aside. In a bowl, beat egg yolks. Stir in lime zest and condensed milk. Stir in water, lime juice. Add green and yellow food coloring to adjust color, if desired. Pour into pie shell. Bake for 30 minutes. Remove from oven and increase temperature to 350F. Meanwhile, for the meringue, in a clean bowl combine, vanilla, cream of tartar and egg whites. Beat with an electric mixer on medium speed for about 1 minute or until soft peaks form. Gradually add sugar, beating on high speed for about 4 minutes or until sugar is dissolved and stiff peaks form. Spead meringue over hot pie, sealing edges of crust. Bake for 15 minutes. Cool 1 hour. Chill for 3 to 6 hours before serving. Keep leftovers refrigerated. Makes 8 servings.