Preheat oven to 350°. Grease a large, deep baking dish with butter. In a large pot of boiling salted water, cook macaroni, stirring occasionally, until just al dente according to package instructions. Drain, then rinse with cold water.
In a large bowl (or same pot you used to cook the pasta), toss pasta and butter; season with salt and pepper. Stir in 2 c. cheddar.
Add half of macaroni mixture to prepared baking dish. Sprinkle Colby-cheddar over pasta mixture, then top with another layer of macaroni mixture (similar to making a lasagna).
In a medium bowl, beat eggs and milk until combined. Pour over macaroni mixture.
Cover baking dish with foil and bake macaroni and cheese until cheese is melted and edges are bubbling, 25 to 35 minutes. Do not overbake! It may be a bit jiggly when you take it out of the oven.
Turn on broiler. Top with remaining 2 c. cheddar. Broil, watching closely, until cheese is melted and golden, about 3 minutes. Let cool slightly to firm up.
