Preheat the oven to 375°F (190°C) and lightly grease a 9x13-inch baking dish with nonstick spray.
Arrange the 4 thin-sliced chicken breasts in a single layer in the prepared baking dish and season both sides with salt and black pepper.
In a medium bowl, beat the 8 oz softened cream cheese with 1/2 cup salsa and 1 oz dry fajita seasoning until smooth and evenly combined.
Spread the cream cheese mixture evenly over the tops of the chicken breasts.
Layer the sliced green and red bell peppers over the cream cheese-topped chicken, then sprinkle 2 cups shredded Colby Jack cheese evenly over everything.
Bake uncovered for 20–25 minutes, or until the chicken reaches an internal temperature of 165°F (74°C); start checking around 20 minutes if the breasts are thin.
Let the bake rest for 5 minutes before serving to allow the juices to settle and the cheese topping to set.
