Heat oven to 350°F. Lightly grease bottom only of 15x10x1-inch pan with shortening. In large bowl, beat butter, 2/3 cup powdered sugar, 2 tablespoons lemon peel, 1 tablespoon lemon juice and 2 drops food color with electric mixer on medium speed until creamy, or mix with spoon. Stir in flour until well blended. Press in pan, using floured fingers (flour fingers several times to prevent sticking).