In a small saucepan, combine 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 the lemon juice, zest and vanilla. Cool quickly by placing pan in a bowl of ice water; stir for 2 minutes. Fold in whipped topping and food coloring if desired. Spoon into dessert dishes. Refrigerate for at least 1 hour before serving.