Thoroughly mix gelatin, 1/2 cup sugar, and salt in saucepan. Beat together egg yolks, lime juice, and water; stir into gelatin mixture.
Cook and stir over medium heat just till mixture comes to boiling. Remove from heat; stir in grated peel. Add food coloring sparingly to tint pale green.
Chill, stirring occasionally, until the mixture mounds slightly when dropped from a spoon.
Beat egg whites till soft peaks form; gradually add 1/2 c sugar, beating to stiff peaks.
Fold gelatin mixture into egg whites. Fold in whipped cream. Pile into cooled baked pastry shell. Chill till firm.
Spread with more whipped cream; edge with grated lime peel. Sprinkle chopped pistachio nuts in center.
Garnish with thinly sliced lime placed in whipped cream mounds around edge of pie.
