Pour 2 cups of cold water into a mixing bowl. Sprinkle the entire packet of gelatin evenly over the surface and let it sit for about 5 minutes to bloom.
Gently heat the gelatin mixture over a double boiler or in the microwave in short bursts (about 15 seconds), stirring frequently until the gelatin is fully dissolved but not boiling.
Add 2 cups of lemonade and 1 cup of tonic water to the dissolved gelatin mixture and stir gently to combine.
Add a few drops of food coloring if desired to enhance the glow or create vibrant colors.
Pour the jello mixture carefully into individual plastic cups, filling about three-quarters full.
Place the cups in the refrigerator for at least 4 hours or until fully set.
When ready to serve, turn off the lights and switch on your black light to watch the jello cups glow.