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.