Beat egg yolks in top of a double boiler. Gradually add 1/2 cup sugar, lime juice, and salt, mixing well. Bring water to a boil. Reduce heat to medium; cook, stirring constantly, until thickened. Remove from heat.
Dissolve gelatin in cold water; add to yolk mixture, stirring until well blended. Stir in lime rind and food coloring. Cool.
Beat egg whites (at room temperature) until foamy. Add remaining 1/2 cup sugar, 1 tablespoon at a time, beating until stiff peaks form. Fold into lime mixture. Spoon 3 tablespoons filling into each tart shell. Chill. Garnish with fresh mint sprigs, if desired.
Substitute 1/3 cup lemon juice and 1 tablespoon grated lemon rind for lime juice and rind. Substitute 1 to 2 drops yellow food coloring for green food coloring. Garnish with frosted grapes, if desired.