Spread butter evenly on one side of each piece of bread. Distribute American cheese slices and bacon equally over 4 slices of bread on the side without butter. Place Colby cheese slices over the bacon, and top with the remaining slices of bread, butter-side out.
Preheat a large skillet or griddle over medium heat. Cook sandwiches, in batches if necessary, for 2 to 3 minutes per side, or until golden and the cheeses are melted. Serve immediately.
Source: ilovecheese - from the kitchen of Mr. Foodr