Pulse bread, parmesan, garlic and 2 tablespoons butter in food processor until coarsely ground. divide crumb mixture into 2 zipper lock freezer bags and freeze.
Bring 4 quarts water to boil in Dutch oven over high heat. Add 1 tablespoon salt and macaroni and cook until barely softened, about 3 minutes. Drain pasta, then spread out on rimmed baking sheet and let cool.
Heat remaining butter, flour, mustard and cayenne in empty pot over medium high heat, stirring constantly, until golden and fragrant, 1 to 2 minutes. Slowly whisk in broth and cream and bring to boil. Reduce heat to medium and simmer until slightly thickened, about 15 minutes. Off heat, whisk in colby, cheddar, 1 teaspoon salt, and 1/2 teaspoon pepper until smooth.
Stir cooled pasta into sauce, breaking up any clumps, until well combined. Divide pasta mixture between two 8 inch square baking dishes. Cool to room temperature, about 2 hours. Wrap dishes tightly with plastic, cover with foil, and freeze up to 2 months.
Adjust oven rack to middle position and heat oven to 375 degrees. Remove foil from casserole and reserve. Microwave casserole until mixture is thawed and beginning to bubble around edges, 7 to 12 minutes, stirring and replacing plastic halfway through cooking. (If preparing both dishes, microwave one at a time.) Discard plastic and cover pan with reserved foil. Bake 20 minutes, then remove foil and sprinkle with 1 bag frozen bread crumbs. (If preparing both dishes, sprinkle second bag of crumbs over second dish). Continue to bake until crumbs are golden brown and crisp, about 20 minutes longer. Let cool 10 minutes. Serve.
Recipe by This Mama Loves at http://www.thismamaloves.com/freezer-macaroni-cheese/