Preheat the oven to 350 degrees F and generously butter a 2-quart shallow baking dish.
In a large saucepan, melt the butter over medium heat. When the foam subsides, add the flour and cook over moderate heat, stirring constantly, for 2 minutes.
Add the half-and-half, whisking, and cook over moderate heat until thickened, about 3 minutes. Add the nutmeg and season with salt and pepper to taste. Add half of the cheddar and half of the Colby-Jack and cook over low heat, stirring, until the cheese is melted. Off the heat, stir in the mustard and paprika.
Meanwhile, cook the pasta in salted boiling water until al dente. Drain well, shaking out any excess water. Return the pasta to the pot along with the cheese sauce and the remaining cheese; stir until evenly combined.
Pour the mixture into the prepared dish in an even layer. Sprinkle the Parmesan over the macaroni and bake for 45 minutes, or until bubbling and golden.
Let rest for 15 minutes before serving.
Make ahead: The unbaked mac and cheese can be refrigerated overnight. Return to room temperature before baking.
"The Portlandia Cookbook: Cook Like a Local" by Fred Armisen, Carrie Brownstein and Jonathan Krisel © Clarkson Potter 2014. Reprinted with permission from Clarkson Potter. All rights reserved.