Line a large baking sheet or tray with parchment paper or aluminum foil.
Place 12 ounces semi-sweet chocolate chips in a microwave-safe bowl. Microwave in 20-second intervals, stirring after each interval, until completely melted and smooth.
Place 4 ounces white chocolate chips in a separate microwave-safe bowl. Microwave in 20-second intervals, stirring after each interval, until melted.
Place 1 cup pink candy melts, 1 cup purple candy melts, and 1 cup yellow candy melts into three separate microwave-safe bowls. Microwave each bowl in 20-second intervals, stirring between each interval, until smooth and melted.
Stir a small spoonful of the melted white chocolate into each bowl of melted candy melts to create softer pastel colors.
Pour the melted semi-sweet chocolate onto the prepared baking sheet and spread it into an even layer with an offset spatula or the back of a spoon.
Drop spoonfuls of the pink, purple, and yellow candy melts over the melted chocolate.
Use the tip of a butter knife to gently swirl the candy melts into the chocolate, being careful not to overmix so the colors remain distinct.
Sprinkle 1/2 cup pastel M&M's and pastel sprinkles evenly over the top.
Refrigerate the bark for 30 to 60 minutes, or until completely firm.
Break the bark into pieces and serve.
Store any leftover bark in an airtight container in the refrigerator.