In a saucepan, combine the sugar and cornstarch; gradually stir in milk until smooth. Bring to a boil over medium heat, stirring constantly. Cook and stir for 2 minutes or until thickened and bubbly. Remove from the heat.
Stir in lemon juice, peel and vanilla. Set saucepan in ice; stir until mixture reaches room temperature, about 5 minutes.
Fold in whipped topping and food coloring if desired.