Wash and thoroughly dry each apple, since any leftover moisture will keep the candy coating from sticking properly later. Insert a candy apple stick firmly into the top of each apple where the stem used to be. Lightly grease your cookie sheet or line it with parchment paper and set it aside.
In your saucepan, combine the sugar, corn syrup, water, and white liquid food coloring, then stir over medium heat until the sugar fully dissolves. Clip your candy thermometer to the side of the pan, making sure the tip doesn't touch the bottom. Bring the mixture to a boil without stirring any further.
As the syrup boils, use your wet pastry brush to wipe down any sugar crystals forming on the sides of the pan. Let it continue cooking undisturbed until the thermometer reads 300 to 302°F, which is the hard crack stage that gives candy apples their signature crunch. This usually takes around 10 to 15 minutes depending on your stove.
Once the syrup reaches temperature, remove the pan from the heat immediately to stop the cooking process. Stir in your gel food coloring and cotton candy flavoring quickly, since the syrup will start to thicken and set as it cools. Work fast here so you still have time to dip all your apples.
Holding each apple by its stick, tilt the pan slightly and dip the apple into the hot syrup, twirling it to coat evenly on all sides. Lift it out and let any excess syrup drip back into the pan for a few seconds. Place the coated apple onto your prepared tray to set.
While the candy shell is still slightly tacky, gently press or wrap a small tuft of cotton candy around the top of each apple. Work quickly since the shell hardens fast and the cotton candy needs something slightly sticky to cling to. Let the apples finish cooling completely at room temperature before serving.