Line a baking sheet or large tray with parchment paper or a silicone baking mat.
In a microwave-safe bowl, melt the white candy melts in 30-second intervals, stirring after each until smooth. Pour the melted candy onto the prepared pan and spread it into an even layer about ¼ inch thick.
Melt the orange, purple, green, and black candy melts in separate microwave-safe bowls, using the same 30-second intervals.
Drop spoonfuls of the colored candy melts over the white layer. Swirl the colors together with a toothpick or skewer to create a marbled design.
While the candy is still warm, sprinkle with candy corn, broken pretzels, and Halloween sprinkles. Gently press the toppings into the candy so they stick.
Let the bark set at room temperature for 1–2 hours, or refrigerate for about 30 minutes until firm.
Break into pieces and serve.