Cook the elbow macaroni according to the package instructions for al dente pasta. The noodles will continue to soften when the mac and cheese bakes. Drain, and set aside.
Preheat the oven to 350°F. In a pot over medium to medium-low heat, melt the butter. Gradually whisk in the flour to make a light, smooth roux. Season with the onion powder, garlic powder, paprika, salt, white pepper, and mustard powder.
Once the roux is smooth, slowly pour in the evaporated milk while whisking continuously. Continue stirring until smooth. When the sauce begins to thicken, reduce the heat to its lowest setting, and stir in ½ cup of heavy cream until smooth.
Combine the mild cheddar, medium cheddar, mozzarella, and Colby Jack. Gradually add about half of the cheese mixture tot he sauce, stirring gently until the cheese has melted and the sauce is smooth. Keep the heat very low during this step to prevent the cheese sauce from breaking.
Pour the cheese sauce over the cooked macaroni, and stir until well combined. Add the sour cream, and stir until thoroughly incorporated. If needed, add additional heavy cream a few tablespoons at a time until the mac and cheese reaches your desired creaminess.
Add about one-third of the mac and cheese to a 9x13-inch casserole dish, spreading it evenly across the bottom. Sprinkle about one-third of the remaining shredded cheese over the first layer. Repeat the layering process two more times, finishing with the remaining cheese on top.
Bake uncovered at 350°F for approximately 35minutes, or until the mac and cheese is bubbly and golden to your liking. For a little more color and a slight crust on top, switch the oven to low broil for the final 3 to 5 minutes, watching closely to prevent burning.
Remove the mac and cheese from the oven, and allow it to rest for at least 10 minutes before serving.