In saucepan, combine one package undrained raspberries and cornstarch; cook and stir until thickened and clear. Chill while making mousse mixture.
In blender, combine remaining package drained raspberries, sweetened condensed milk and lemon juice; blend until smooth. Place in bowl; stir in small amount of food coloring if desired. Fold in whipped topping.
Spoon half mousse mixture into dessert dishes; top with equal portions of raspberry sauce then remaining mousse mixture. Chill at least 1 hour. Refrigerate leftovers.