Prepare the Pan: Line an 8×8-inch pan with parchment paper, covering the bottom and sides.
Melt Ingredients: In a large microwave-safe bowl, combine white chocolate chips, sweetened condensed milk, butter, and peppermint extract. Microwave for 30 seconds, stir, and repeat in 30-second intervals until smooth. Be careful not to overheat.
Add Colors: Divide ½ cup of the mixture into each of two small bowls. In one, add red food coloring, stirring quickly to mix. In the other, add green food coloring, stirring until evenly combined.
Create Swirls: Spoon dollops of the plain, red, and green fudge mixtures into the prepared pan, distributing them randomly. Gently swirl with a spoon or butter knife to create a marbled effect. Add sprinkles if desired.
Chill: Refrigerate the fudge for at least 2 hours to set.
Slice and Serve: Lift the fudge out of the pan using the parchment paper, and cut into squares with a sharp knife, wiping the blade between cuts for clean edges.
Enjoy your holiday-inspired fudge!