Preheat the oven to 350°F. Lightly grease a 9x13-inch baking dish.
Bring a large pot of salted water to a boil. Cook the macaroni until just al dente, then drain well. Do not rinse.
While the pasta is still warm, add the butter and softened cream cheese. Stir until melted and evenly incorporated.
In a separate bowl, whisk together the evaporated milk, 1 cup heavy cream, eggs, black pepper, and nutmeg until smooth.
Pour the milk-and-egg mixture over the warm macaroni and gently stir to combine. Add the Velveeta, mozzarella, 2 cups sharp cheddar, and 1 cup Colby Jack. Gently mix until everything is evenly distributed.
Pour the macaroni mixture into the prepared baking dish. Sprinkle the remaining ½ cup sharp cheddar and ½ cup Colby Jack evenly over the top. Pour the remaining ½ cup heavy cream evenly over the macaroni.
Cover tightly with foil and bake for 25 minutes. Remove the foil and bake for another 15 minutes, or until the top is golden and the center is hot and creamy.
Remove from the oven and let rest for 10-15 minutes before serving.