8 clean wooden twigs; a candy thermometer; a wooden skewer
Wash and dry the apples and remove the stems. Insert a wooden twig into each apple about halfway through. Line a baking sheet with parchment paper.
Add the sugar, corn syrup, 1/2 teaspoon of the blue food coloring, 1/2 teaspoon of the red food coloring and 1 cup water to a small saucepot and stir together. Attach a candy thermometer to the pot and bring the mixture to a boil. (Do not stir once the sugar starts to boil.) Boil until the mixture reaches 295 degrees F, 15 to 20 minutes. Immediately remove from the heat and add the remaining 1/2 teaspoon blue food coloring and 1/2 teaspoon red food coloring, using a wooden skewer to swirl a figure eight in the pot and being careful not to blend.
Immediately dip the apples into the mixture, carefully tilting the pot as necessary to coat. Allow any excess coating to drip back into the pot, then place the apples on the parchment-lined baking sheet to set, about 1 hour.
To add even more marbling, use a small brush to speckle gel food coloring on the set apples.