Place sanding sugars in 4 bowls. Chop white chocolate. Microwave two-thirds of it at 50% power, stirring every 30 seconds, until smooth, about 2 minutes. Stir in remaining chocolate to melt. Drop a mini pretzel into the chocolate and turn with a fork to coat. Lift pretzel out, tapping fork on the edge of the bowl to remove excess, and transfer to one of the sugars; turn to coat. Place on wax paper. Repeat with remaining pretzels, reheating the chocolate in the microwave as needed. Refrigerate to set.