In a large saucepan, combine sugar, corn syrup, water and food coloring. If any sugar granules end up on the inside of the pan, use a wet pastry brush to clean them down. Set the mixture over medium to high heat and bring to a boil. Insert a candy thermometer into the liquid to allow it to boil, without stirring, until it reaches 302°F (hard crack stage), about 20 minutes. Remove from heat.
Add vanilla and stir only slightly to blend. Working with 1 at a time, dip the apples into the mixture, tilting the pan to coat them evenly. Let the excess dip off into the pan, then set the coated apples on a baking sheet lined with parchment paper. Allow to dry for 30 minutes before eating.
Recipe Notes
Store covered in a cool, dry place. Do not refrigerate, or the candy coating will become soft.