Pop the popcorn in your air popper if you haven’t already. Place the popcorn in an oven-safe bowl.
Place the popcorn in an oven preheated to 200 degrees Fahrenheit to keep it warm while you make the syrup.
Combine the granulated sugar, agave nectar, coconut oil, salt, and cream of tartar in a saucepan.
Bring the sugar mix to a boil. Boil it until a candy thermometer dipped into the syrup reads 250 degrees Fahrenheit (121 degrees Celsius). Do not stir.
Remove the syrup from the heat and stir in the baking soda. It will likely fizz a bit as the baking soda reacts with the cream of tartar.
If adding color to the syrup, divide it among multiple bowls, depending on the number of colors you want. Add a few drops of natural food coloring to each bowl and stir, making sure to rinse the spoon between bowls.
Remove the popcorn from the oven. If you added color to the syrup, it may also help to divide the popcorn into a few separate bowls to get a good mix of the distinct colors.
Pour the syrup over the popcorn. (Use one syrup color per bowl of popcorn if you’re using multiple colors.) Stir well to coat the popcorn with the syrup.
Spread the popcorn on cooling racks lined with parchment paper. Allow to cool. The syrup will naturally dry and harden as it cools, forming the candy coating for the popcorn.
Pour this popcorn into a serving bowl.