Line a 9x13-inch baking sheet with parchment paper, letting the edges hang over the sides for easy removal later.
Place dark chocolate in a microwave-safe bowl and microwave in 30-second intervals at full power, stirring after each burst, until completely melted and smooth. This should take 1.5 to 2 minutes total.
Pour the melted dark chocolate onto the prepared baking sheet and use an offset spatula to spread it into an even layer about 1/4-inch thick.
In a separate microwave-safe bowl, melt the white chocolate using 20-second intervals at 50% power, stirring between each interval until smooth. White chocolate burns easily, so watch it carefully.
Divide the melted white chocolate into 2-3 small bowls depending on how many colors you want. Add 2-3 drops of gel food coloring to each bowl and stir until the color is evenly distributed and vibrant.
Drizzle the colored white chocolate over the dark chocolate base in random lines and swirls, alternating colors across the surface.
Immediately drag a toothpick, skewer, or butter knife through the colored chocolate in various directions — make figure-eights, zigzags, or straight lines to create a marbled tie-dye effect.
Tap the baking sheet firmly on the counter 3-4 times to release any air bubbles and smooth the surface.
Refrigerate for 15-20 minutes or freeze for 10 minutes until the chocolate is completely set and firm to the touch.
Lift the bark out using the parchment paper overhang, then break it into irregular pieces using your hands. Store in an airtight container.
