In a small saucepan over medium heat, combine lemon juice, sugar, lemon zest & salt, stirring well to dissolve sugar. Add the cornstarch mixture & the milk, stirring well to combine. Cook until mixture begins to thicken, about 5-6 minutes, stirring constantly until the first few bubbles appear on the surface. Add butter & food color; continue to cook until resembles thick pudding. Transfer to a dish; cover with plastic wrap & allow to cool completely before placing in refrigerator. Chill at least 2 hours in the refrigerator or preferably overnight.