Bring a large pot of salted water to a boil. Add the pasta and cook as the label directs. Reserve 1/2 cup cooking water, then drain.
Combine the evaporated milk, eggs, havarti, Colby jack, 1/2 teaspoon salt and the nutmeg in a large saucepan over medium heat. Cook, whisking constantly, until smooth and thickened, about 8 minutes. Remove from the heat.
Add the pasta and sour cream to the sauce; toss to coat, then let sit 5 minutes so the sauce thickens slightly. Add the reserved cooking water to loosen, if desired.