Stir together sugar and 1 cup water in small saucepan. Heat over medium flame and simmer until the sugar is welldissolved. Sprinkle gelatin over, stir, let sit 2 minutes, and stir again. Add lime juice and cold water, and food coloring if using, stir, and pour into 8 by 8-inch pan. Refrigerate until firm, 4 to 6 hours, and cut into cubes.
These citrus gels need extra gelatin to compensate for their acidity, which weakens setting.
c.1997, M.S. Milliken & S. Feniger, all rights reserved