Begin by washing your green or red grapes thoroughly under cold water. Dry them completely using a clean kitchen towel or paper towels.

In a medium saucepan over medium heat, combine granulated sugar, water, and light corn syrup. Stir until the sugar fully dissolves and bring it to a simmer until it reaches around 300°F.

Once the mixture reaches the right temperature, carefully divide it into three separate bowls. Add white gel food coloring to one bowl, and pink and blue gel colors to the others. Stir until smooth.

Prepare a baking sheet lined with parchment paper. Using a fork or skewer, dip each grape into the colored mixtures, swirling them to create a marbled effect.

Let the dipped grapes sit at room temperature for about 30 minutes to harden, ensuring the candy coating turns glossy and firm.