In a small saucepan, combine the clementine zest with 1/2 cup of water and 1/2 cup of sugar. Bring the mixture to a boil over medium heat until the sugar has dissolved, then turn off the heat and let cool.
Combine the juice from the clementines, the juice of 1 lime and the cooled simple syrup in a large pitcher. Stir to combine. Add tequila, if desired.
In a resealable bag combine the remaining 2 tablespoons of sugar with the food coloring. Shake and massage the bag to combine the food coloring with the sugar.
On a flat plate, combine the colored sugar and salt and spread evenly across the plate. Dip the rims of 4 margarita glasses in water and then in the sugar mixture. Fill the glasses with ice and evenly distribute the margaritas among the glasses. Garnish with slices of lime and serve.
Recipe courtesy of Sandra Lee