Prepare the oven and baking dish.
Preheat the oven to 375°F. Lightly grease a 9-by-13-inch baking dish.
Cook the pasta.
Bring a large pot of water to a boil and add 1 teaspoon salt. Add the elbow pasta and cook until just shy of al dente, following the package timing as a guide. It will continue cooking in the oven.
Drain without rinsing and set aside.
Prepare the cheese.
Shred the cheddar and Colby Jack, and shred or tear the American cheese into small pieces. Reserve about 1 cup of the combined cheeses for the topping.
Make the sauce base.
In a large pot over medium-low heat, melt the butter. Whisk in the flour and cook for 1–2 minutes, stirring constantly, until smooth and lightly bubbling.
Gradually whisk in the milk until no lumps remain.
Add the paprika, seasoning salt, onion powder, garlic powder, and crumbled bouillon cube. Cook gently, stirring, until the bouillon dissolves and the sauce thickens slightly.
Melt in the cheese.
Reduce the heat to low. Add the cheese a small handful at a time, stirring until each addition melts before adding more. Keep the reserved cheese aside.
Stir in the heavy cream, then remove from the heat. Taste before adding any extra salt and black pepper.
Assemble.
Place the drained pasta in the prepared baking dish. Pour the cheese sauce over it and gently stir until all the pasta is coated.
Sprinkle the reserved cheese evenly over the top.
Bake.
Cover with aluminum foil and bake for 20–25 minutes, until hot and bubbling around the edges.
Remove the foil and bake for another 5–10 minutes, until the cheese on top is melted and lightly golden. Watch closely so it doesn’t become too dark.
Serve.
Let rest for about 5 minutes, then serve warm while the sauce is creamy and the cheese is melty.