Preheat oven to 325°F and grease a 9x13 inch baking dish or cast iron skillet.
Cook macaroni according to package directions until just al dente, drain and set aside.
In a large bowl, whisk together eggs, milk, heavy cream, melted butter, salt, black pepper, and cayenne pepper.
Add the cooked macaroni to the bowl and stir to coat with the egg mixture.
Fold in 3 cups of cheddar cheese and 1.5 cups of Colby cheese, reserving the rest for topping.
Pour mixture into prepared baking dish and top with remaining cheese.
Bake for 45-50 minutes until the top is golden brown and the center is set.
Let rest for 10 minutes before serving to allow the custard to firm up.