Heat the white and milk chocolate in microwave safe dishes at 30-second intervals until smooth and melted. Spoon these two onto a parchment lined rimmed baking sheet while heating the orange and purple. Mix together using an offset spatula and then dollop the purple and orange on top, swirling with a toothpick or skewer.
While the chocolate is still tacky, gently press the pretzels pieces in and then top with desired sprinkles.
Place in the refrigerator for 5-10 minutes, or until set. Remove and roughly cut with a large knife or your hands (I prefer this method). Do not worry about cutting in nice clean squares, bark is best in rough, uneven pieces.
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Notes
Tip: If chocolate starts to set too fast, place it in a 300°F oven for 30 seconds to allow chocolate to melt again.