Preheat oven to 350°F Prepare and bake cake mix as package directs for 13x9-inch cake.
In medium bowl, beat sweetened condensed milk and egg yolks. Stir in lemon juice, lemon rind (optional) and food coloring (optional). Spread evenly over hot cake. Return to oven; bake 8 minutes longer. Cool. Chill. Garnish as desired. Store leftovers covered in refrigerator.