1. Preheat the oven to 350°. Generously butter a shallow 2-quart baking dish. Melt 3 tablespoons of the butter in a large saucepan. Add the flour and cook over moderate heat for 2 minutes, stirring constantly. Add the half-and-half and cook over moderate heat, whisking constantly until thickened, about 3 minutes. Add one-half of the Cheddar and Colby cheeses and cook over low heat, stirring, until melted. Stir in the mustard, nutmeg and cayenne; season with salt and pepper.
2. Meanwhile, cook the elbow macaroni in a large pot of boiling salted water until al dente. Drain very well. Return the macaroni to the pot. Add the cheese sauce and the remaining cheese and stir until combined. Spread the macaroni in the prepared baking dish.
3. In a small glass bowl, melt the remaining 2 tablespoons of butter in a microwave oven. Add the bread crumbs, season with salt and pepper and stir until evenly moistened. Sprinkle the buttered crumbs over the macaroni and bake for 45 minutes, or until bubbling and golden on top. Let stand for 15 minutes before serving.
Make Ahead: The assembled dish can be refrigerated overnight. Bring to room temperature and bake.