Bring a large pot of salted water to a boil; add the macaroni and cook until al dente. Drain and return to the pot. Preheat the oven to 350°F. Using the nonstick spray, oil, or butter, grease a 2-quart casserole dish. In a medium saucepan over medium heat, bring the milk, butter, and mustard to a simmer. Whisk in the flour. Continue to cook, stirring constantly to prevent burning, until the mixture is thick and bubbly, about 5 to 7 minutes. Reduce the heat, add the processed cheese, and stir until completely melted. Pour the cheese sauce and shredded Cheddar cheese over the cooked pasta in the pot, stirring to evenly coat.
Transfer half of the pasta mixture to the prepared casserole dish. Top with an even layer of the Colby Jack cheese slices. Top the cheese slices with the remaining pasta mixture. Bake, uncovered, until the top is golden brown, about 30 minutes. Let stand 10 minutes before serving.