1) In a small pot on medium heat dissolve sugar in water. Raise the heat, add the lime rind and bring to a boil. Lower the heat to a simmer and cook until slightly reduced. Remove from heat and discard the lime rinds. Stir in the food coloring and cool.
2) Fill 4 glasses with ice and distribute the lime syrup and lime juice, to taste, into each glass. Top with seltzer and stir. Garnish with lime slices.