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 lime juice and, if desired, food coloring. Bring to a boil over medium heat, whisking constantly. (Mixture will begin to thicken when lime 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 lime 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).