Place the 12 stripped corn cobs in a large pot with 4 cups of water. Bring the water to a boil, reduce the heat, and simmer the cobs for 30 to 40 minutes.

Strain the cooked corn liquid through a jelly bag, fine-mesh strainer, or several layers of cheesecloth. Do not press or squeeze the cobs or jelly bag if you want a clear finished jelly, and measure exactly 3 cups of liquid.

Pour the measured 3 cups of corn liquid into a clean pot and stir in the powdered fruit pectin. Bring the mixture to a full rolling boil over medium-high to high heat, stirring frequently.

Add all 4 cups of sugar at once and stir until dissolved. Return the mixture to a full rolling boil and boil hard for 1 minute, stirring constantly.

Remove the pot from the heat and skim off any foam. Add a few drops of red or yellow food coloring, if desired, and stir gently.

Ladle the hot jelly into hot, sterilized half-pint jars, leaving about 1/4 inch of headspace. Wipe the rims clean, apply the lids and rings, and process the jars in a boiling-water canner for about 10 minutes, adjusting for altitude according to standard canning guidance.

Remove the jars and let them cool undisturbed. Once cool, check that each lid has sealed before labeling and storing the jars.