Meanwhile, prepare the pretzels. Line a baking tray with wax paper. Place the pretzels on the tray. Pour candy melts and vegetable shortening in a microwave-safe bowl. If a darker mossy-colored green is desired, add a drop or two of black candy coloring. Microwave for 90 seconds at 70 percent power. Stir and continue to microwave at 30-second, then 15-second intervals, until fully melted. Stir well, then use a small spoon to drizzle green candy melts onto the pretzels, covering as much or as little as desired. You may not use all of the melts; if desired, use leftover melted candy melts to dip extra pretzels to serve alongside your brownies.