The night before cooking: Pat turkey skin completely dry with paper towels and sprinkle salt lightly all over. Rub about 1 teaspoon salt inside walls of turkey cavity.
To make chile Colorado sauce: Preheat broiler. Put onion, tomatillos, tomatoes and garlic on a baking pan and drizzle them with olive oil.
Put baking sheet under broiler and cook without turning until vegetables start to charr, about 7 minutes. Remove, set aside, and let cool to room temperature.
In a large dry skillet over medium-low heat, toast guajillos, turning them over halfway through, until aromatic, about 1 minute.
Transfer them to a bowl, cover them with hot water and let them soak until they’re soft, about 30 minutes. Drain chiles and discard soaking water.
Combine vegetables and chiles in a blender with chicken stock (work in batches) and puree until mixture is very smooth.
Transfer each batch to a bowl as it’s done, and stir batches together well. Season with salt and pepper to taste.
Rub 1/2 cup of Chile Colorado Sauce all over turkey, reaching under the skin to massage it into breast meat and rubbing a few spoonfuls inside the cavity.
Wrap flap of neck skin down over the cavity, tucking it under the body, and tuck wing tips up under breast.
Set turkey on a rack in a roasting pan and cover it loosely with a plastic wrap. Let marinate overnight in refrigerator.
Preheat oven to 350°F.
At least 1 hour before cooking, remove turkey from refrigerator and let stand uncovered until it comes to room temperature.
Position oven rack in the lower third of oven. While oven is heating, rub 2 tablespoons butter over the skin of turkey. Pour a cup of water into bottom of roasting pan.
Roast turkey for 1 hour.
Remove bird from oven and brush it all over with 1/2 cup of Chile Colorado Sauce.
Pour another cup of water into bottom of pan. Tent turkey loosely with a sheet of foil and return it to oven.
Roast turkey until an instant-read thermometer inserted into the thickest part of the thigh reaches 170°F, 2 to 3 hours more. Check it occasionally after 2 hours. If bottom of pan gets dry during roasting, add a little more water, 1/2 cup at a time.
Transfer turkey to a serving platter and let rest for 30 minutes. (During this time, the temperature in the thigh will rise to 175°F).
To make gravy: Pour the pan juices into a heatproof glass 2 cup measure.
Add 1/2 cup of chicken stock to juices in roasting pan and swirl it, scraping up any browned bits with a wooden spoon.
Pour it all into the measuring cup. Skim off the fat, reserving it in a separate cup. If pan juices and chicken stock do not make 2 cups, add more chicken stock to make 2 cups.
Put 6 tablespoons of reserved fat in a large heavy saucepan over medium heat. If there isn't enough fat, add butter until you have 6 tablespoons total.
Whisk in flour and remaining 1/2 cup chile Colorado sauce, and cook over medium heat, whisking continually for 3 minutes, until mixture is very thick.
Pour in pan juices, whisking constantly to avoid lumps. Whisk in remaining chicken stock and bring mixture to a boil.
Reduce heat and simmer, whisking now and then, until gravy is thick and smooth, 12 to 15 minutes. Season with salt to taste.
Serve turkey with gravy.