Whisk together 3/4 cup milk and 1/4 cup cornstarch in a 3-quart heavy saucepan, whisking until cornstarch dissolves.
Whisk in sweetened condensed milk and eggs until blended; whisk in lemon juice and, if desired, food coloring. Bring to a boil over medium heat, whisking constantly. (Mixture will begin to thicken when lemon juice is first added, and then become thin again during first few minutes of cooking. It will thicken quickly as it comes to a boil.)
Boil 1 minute, whisking constantly, or until mixture thickens. Remove mixture from heat, and whisk in butter and lemon rind until smooth. Pour filling into a large bowl, and place bowl in a larger bowl filled with ice. Stir regularly until cold (about 10 minutes).
Lime Icebox Pie Filling: Substitute 1/2 cup fresh lime juice for lemon juice, 1 drop green food coloring and 1 drop yellow food coloring for the 3 drops yellow food coloring (optional), and 1 tablespoon grated lime rind for lemon rind.
Orange Icebox Pie Filling: Substitute 1/2 cup thawed frozen orange juice concentrate for fresh lemon juice, 2 drops red food coloring and 4 drops yellow food coloring for 3 drops yellow food coloring (optional), and 1 tablespoon fresh grated orange rind for lemon rind.