1. Thoroughly mix the gelatin, sugar, and salt in a saucepan.
2. Beat together the egg yolks, lime juice, and water; stir into the gelat mixture. Cook over medium heat, stirring constantly, just until the mixtur comes to a boil. Remove from the heat; stir in the grated peel.
3. Add the food coloring sparingly to give a pale green color. Chill, stirring occasionally, until the mixture mounds slightly when dropped from a spoon.
4. Beat the egg whites until soft peaks form; gradually add 1/2 cup sugar, beating to stiff peaks.
5. Fold the gelatin mixture into the egg whites; fold in the whipped cream Pile into the pastry shell. Chill until firm.
6. Spread with additional whipped cream and edge with grated lime.