In a small saucepan combine sugar and cornstarch. Stir in milk. Cook and stir milk mixture over medium heat until thickened and bubbly. Reduce heat. Cook and stir milk mixture for 2 minutes more. In a small mixing bowl beat egg and egg yolk. Gradually stir about 1/2 cup of the hot milk mixture into the beaten egg mixture.
Return milk and egg mixture to saucepan. Cook and stir until nearly bubbly. Do not boil. Cook and stir mixture for 2 minutes more. Remove from heat.
Stir in lime juice, butter, and food coloring. Cover surface of custard with plastic wrap. Cool completely. Fold custard into yogurt. Cover and chill.
To serve, cut angel cake or pound cake into 8 slices. Place one slice each on 8 dessert plates. Spoon lime custard onto each piece. Top with sliced fruits. Makes 8 servings.