Heat the oven to 350°F and grease a 2-quart baking dish. Cook macaroni in a large pot of water with the tablespoon salt until just tender, then drain.
Toss the pasta with melted butter and let cool for 5 minutes until warm rather than steaming. Combine cheddar and Colby in a separate bowl.
Whisk eggs, evaporated milk, whole milk, custard salt, pepper and paprika until smooth. Fold three-quarters of the cheese into the pasta, then stir in the egg mixture.
Transfer to the dish and scatter the remaining cheese over the surface. Bake for 30 to 35 minutes until the middle reaches 160°F and no liquid custard moves when the pan is gently nudged.
Let stand for 10 minutes so the custard settles, then serve in soft wedges or spoonfuls.