Heat oven to 350 degrees F. Lightly grease bottom only of 15 x 10 x 1-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).
Bake 20 to 25 minutes or until light golden brown.
In medium bowl, mix all glaze ingredients until smooth and spreadable. Pour glaze over warm bars; spread evenly (glaze will be very thin). Cool completely, about 1 hour. Bars are very tender; remove from pan using very flat-edged metal pancake turner. For bars, cut into 8 rows by 8 rows.
To cut diamond shapes, cut parallel lines, 1 1/2 inches apart, down length of pan, then cut diagonal lines, 1 1/2 inches apart, across straight cuts.