Whisk together sugar and cornstarch in a heavy, nonaluminum, medium saucepan; gradually whisk in lemon juice, eggs, and butter. Cook mixture, whisking constantly, over medium-low heat 8 to 12 minutes or until thick and bubbly. Remove from heat, add food coloring, and let stand 10 minutes.
*1/2 cup thawed lemon juice may be substituted.
Note: You may double this recipe and cook it in one saucepan if you need two recipes of the filling. For testing purposes only, we used Minute Maid 100% Pure Lemon Juice From Concentrate. It can be found with the other frozen juices and lemonades at your grocery store.