Add the gel food coloring, beginning with 1 or 2 drops.
Seal up the plastic bag and start kneading the food gel into the sugar.
After the color is even distributed, add the ¼ teaspoon of cornstarch and continue to knead until everything is incorporated.
Once everything is distributed evenly, spread the colored sugar onto a baking sheet and let it dry for about 30 minutes.
If you find that after the sugar dries there were still small "clumps" of the gel, place all the sugar into a sieve and pushed it through with a spoon.