Heat oven to 350 degrees. Grease a 13x9 baking dish.
Bring 4 quarts of water to boil in a large pot. Add 1 T salt to the water. Add the pasta, occasionally stirring, until the pasta is nearly al dente. (The pasta will finish cooking in the oven.) Drain the pasta and set aside.
Make sure the pot is dry. Add the butter to the to the pot, and melt over medium heat. Stir in the garlic, mustard, and cayenne pepper. Cook until fragrant, about 30 seconds. Add the flour, stirring for 1 minute. Slowly add the milk and broth, stirring with a whisk until smooth. Simmer the mixture until thickened, whisking often, about 15 minutes. Once thickened, remove from the heat.
Gradually whisk in all of the Colby cheese and 2 cups of the cheddar cheese until melted and smooth. Stir in the cooked pasta, breaking up the pasta that sticks together. Pour the mixture into the prepared baking dish.
Sprinkle the remaining 2 cups of cheddar cheese over the top.
Bake for 20 to 25 minutes, or until the top is golden and the edges are bubbly.
Allow to cool for 10 minutes before serving.