Preheat the oven to 375° F.
Bring a large pot of water with a pinch of salt to a boil. Add pasta and cook according to the package directions. Rinse the pasta with cold water, drain well, and set aside.
Meanwhile, if you haven't already, start shredding the cheeses. The three cheddar cheeses can go together, but keep the colby jack cheese separate. Set aside.
Heat the milk in a small saucepan, but don't boil it. Keep it warm.
Make a light roux by melting 8 tablespoons (1 stick) of butter in a large pot over medium-low heat. Then, add the flour and cook on low heat while stirring with a whisk constantly for 3 minutes—it should be a cream color and puff slightly.
While whisking, add the hot milk and cook for 2-5 minutes until thickened to a smooth, creamy consistency. Tip: Coat a spoon in the sauce, then draw a line across the back of the spoon with your fingertip. If the line remains clear, the sauce is ready.
Turn off the heat and add the freshly shredded medium, sharp, and extra-sharp white cheddar cheese. Mix in the seasonings. Then, add the cooked pasta and stir well until the pasta is thoroughly coated with the cheese sauce. Taste the mac' n cheese and season with salt and pepper to taste if needed. Pour half of the mac' n cheese into a 9x13 baking dish. Spread 1 1/2 cups of the shredded Colby Jack cheese evenly, then pour the remaining mac' n cheese on top.
In a small bowl, melt the remaining 2 tablespoons of butter. Mix in the crushed pretzels, Italian-style panko breadcrumbs, and remaining 1/2 cup shredded Colby jack cheese. Sprinkle the pretzel mixture evenly on top of the mac n cheese.
Place the dish in the middle rack of the oven and bake uncovered for 30 - 35 minutes or until the top is golden brown. Serve immediately and enjoy!