Cook the shells in well-salted water until just past al dente, a little softer than you'd cook pasta for an Italian dish. Drain but don't rinse, and don't toss with butter.

While the pasta drains, combine the evaporated milk, whole milk, and canola oil in the same pot you used to boil the pasta. Place the pot over medium-low heat and whisk gently until just steaming.

Add the American cheese and stir until completely melted and smooth—the American cheese is doing the emulsifying work here. Reduce the heat to low and add the grated cheddar in two portions, stirring after each addition until smooth. Add the salt, vinegar, and turmeric and mix well.

Taste the sauce. It should be mild and lightly tangy. If it's too sharp from the cheddar, add a splash more evaporated milk.

Fold the drained pasta into the sauce until well-coated. Let it sit for 1 minute off the heat; the sauce will thicken slightly and cling to the shells. Serve immediately.