In a small bowl, combine the butter, mustard, onion, poppy seeds, and sugar. Place roll bottoms cut side up on ungreased baking pan. Top each with colby cheese, ham and mozzarella. Drizzle with half the butter mixture.
Replace roll tops. Drizzle with the remaining butter mixture. Bake, uncovered, at 350 for 10-15 minutes or until cheese is melted.