01 - Line a large baking sheet with parchment paper or a silicone mat. Insert a wooden stick firmly into the stem end of each apple, ensuring it's secure for dipping.
02 - In a heavy-bottomed saucepan, combine sugar, water, and corn syrup. Place over medium heat and stir continuously until the sugar completely dissolves.
03 - Increase heat to high and bring mixture to a rolling boil. Stop stirring and cook until syrup reaches 300°F (150°C) on a candy thermometer. Remove from heat immediately.
04 - Stir in vanilla extract. Quickly add several drops each of red, blue, and white gel food colorings. Gently swirl with a wooden skewer to create visible streaks—avoid fully mixing for distinct marbling.
05 - Hold apples by sticks and dip one at a time into the hot syrup. Rotate to coat evenly, allowing excess to drip off. The swirling motion will enhance the marble pattern on each apple.
06 - Transfer coated apples to the prepared baking sheet. Briefly swirl syrup between dips to maintain the marble effect. Let apples cool completely until the candy shell hardens, approximately 15 minutes.