To prepare the filling: In a small bowl, gradually whisk together sweetened condensed milk, finely shredded lime peel, lime juice, and, if desired, food coloring. Cover and chill about 2 hours or until mixture mounds slightly.
Spoon thickened filling into phyllo dough shells (about 1 tablespoon per shell). If desired, top tarts with dessert topping and quartered lime wedges.
Tip: To get ½ cup lime juice, squeeze 10 to 12 Key limes or 4 to 6 Persian limes. Or use bottled Key lime juice.
To make ahead: Prepare as directed, except do not top with dessert topping or lime wedges. Cover and chill filled tarts for up to 8 hours. If desired, top tarts with dessert topping and lime wedges.