Heat the oven to 350 degrees F. Butter a 9 by 13 inch baking dish well. Cook the macaroni in a large pot of salted water until just shy of al dente, about 1 minute less than the package directs. Drain and set aside.
While the pasta cooks, melt the butter in a large saucepan or Dutch oven over medium heat. Whisk in the flour and cook, whisking constantly, for 1 minute, until the mixture is smooth and smells slightly nutty, not raw.
Pour in the warm milk about a cup at a time, whisking after each addition until smooth. Cook for 4 to 5 minutes, stirring often, until the sauce thickens enough to coat the back of a spoon. Do not let it boil hard.
Remove the pan from the heat. Stir in the salt, garlic powder, smoked paprika, dry mustard, and black pepper until evenly mixed.
Stir in 3 cups of the cheddar and all of the Colby Jack, a handful at a time, letting each addition melt before adding the next. Stir in the sour cream if you are using it.
In a small bowl, whisk the beaten eggs with about 1/2 cup of the warm cheese sauce to temper them. Pour the egg mixture back into the pot and stir everything together quickly.
Add the drained macaroni to the sauce and fold until every noodle is coated. Pour it all into the prepared baking dish and spread it out evenly. Top with the remaining 1 cup of cheddar and a light dusting of smoked paprika.
Bake uncovered for 30 to 35 minutes, until the top is deep golden and the edges are bubbling. For an extra crisp top, broil for the final 1 to 2 minutes and watch it closely.
Rest for 10 minutes before serving. This short rest lets the custard set, so the first slice comes out clean and the sauce stays where it belongs, on the pasta.
