Stir together crushed pretzels and sugar. Stir in melted margarine or butter. Spread mixture evenly into a 9-inch pie plate. Press onto bottom and sides to form a firm, even crust. Bake in a preheated 375 degrees F oven for 5 minutes or till edge is lightly browned. Cool on a rack.
FOR THE FILLING:
In a large mixing bowl combine sweetened condensed milk, limeade concentrate, tequila, orange liqueur, and, if desired, food coloring. In a medium mixing bowl beat whipping cream till soft peaks form; fold into tequila mixture. Spoon filling into crust. Cover and freeze about 4 hours, or till firm. Let stand 10 minutes before serving. If desired, garnish with lime slices.