Preheat oven to 350°F (175°C).
In a medium skillet over medium heat, add ¼ cup olive oil, 2 tablespoons all-purpose flour, and 3 tablespoons chili powder. Whisk constantly until the mixture darkens slightly and becomes fragrant.
Slowly whisk in 1 (15-ounce) can tomato sauce and 1 ½ cups vegetable stock. Add ½ teaspoon garlic powder, ½ teaspoon onion powder, ¼ teaspoon cumin, and salt and pepper to taste. Continue whisking until the sauce begins to thicken.
Remove the skillet from heat and pour out and reserve 1 ½ cups of the sauce into a separate bowl.
Return the skillet with the remaining sauce to low heat and add 2 cups cooked and diced chicken. Stir until the chicken is evenly coated and warmed through; remove from heat.
If your tortillas are stiff, warm them briefly so they are easy to roll.
To assemble, place 8–10 tortillas on a work surface. On each tortilla put about 2 tablespoons of shredded Colby Jack cheese and a portion of the chicken-and-sauce mixture in the center.
Roll each filled tortilla and place seam-side down in a 9×13-inch baking dish, arranging them snugly in the dish.
Pour the reserved 1 ½ cups sauce evenly over the rolled enchiladas, then sprinkle the top with the remaining shredded Colby Jack cheese.
Bake in the preheated oven for 20–25 minutes, or until heated through and bubbly. Let the enchiladas rest a few minutes before serving.