Boil water in a large pot, then cook the 2 cups of dry macaroni. Drain. Turn on the broiler. Heat a cast-iron skillet over medium-low heat. Once hot, add the butter. When the butter is fully melted, sprinkle in the flour. Whisk together until the ingredients are fully mixed. While whisking, slowly pour in the milk. Do a small amount at a time while whisking. The mixture will get super thick at first, do not fear! As you continue to slowly pour all the milk in, it will become a creamy white sauce. Sprinkle in 2 cups of the Colby cheese, whisk together until the cheese is fully melted. Turn off the heat. Mix in the cooked macaroni. Spread the macaroni and cheese evenly in the cast-iron skillet. Sprinkle the last 1/2 cup of Colby cheese on top. Place the cast-iron skillet in the broiler for 5 minutes.