Whisk together the cold water and cornstarch in a small bowl to form a slurry.
Whisk together the corn syrup, cornstarch slurry, lime juice, lime zest, and green gel food coloring in a medium saucepan over medium heat.
Bring to a simmer and then cook for 30 seconds, whisking constantly. Remove from the heat and cool to room temperature.
Rub the rim of a margarita glass with a lime wedge. Dip the glass in the salt.
Shake or stir together the tequila, lime juice, and Green Goo, and pour into the salt-rimmed glass. Serve.
Notes
The green goo will yield about 1 cup - which is enough for 5 syringes with 1 1/2 oz in each. The nutrition info listed below does NOT include the green goo. You will only need 1 syringe per cocktail unless you'd like an extra for garnish.
Use gel food coloring for the green goo, not liquid food coloring. Gel will produce a more vibrant color and won't make it so runny.
Be careful when handling Green Goo because it can stain hands, clothes, and surfaces.