Line a rimmed baking sheet with parchment paper or a silicone baking mat.
Melt the white candy melts in 30-second intervals in the microwave, stirring until smooth. Spread the mixture evenly on the prepared baking sheet, making it about 1/4 inch thick.
Melt each of the colored candy melts separately. Transfer them to piping bags or zip-top bags with the corners snipped.
Pipe the colors over the white base in zig-zag lines. Use a butter knife to swirl the colors together into a marbled design.
Immediately top with candy corns, pretzel pieces, and sprinkles. Press lightly to stick.
Let the bark sit at room temperature for 1-2 hours or refrigerate for 30 minutes.
Break into pieces and store in an airtight container until ready to serve.