Heat oven to 350°. Coat a 13 x 9-inch baking pan with nonstick cooking spray.
Heat a large nonstick skillet over medium-high heat; coat with nonstick cooking spray and add red and green peppers, scallions and garlic. Cook for 5 minutes, stirring frequently, until peppers soften. Set aside.
In a large saucepan, melt butter over medium-high heat. Whisk in flour and cook 1 minute; gradually whisk in milk and bring to a boil. Cook 2 minutes or until thickened. Turn off heat and stir in salt, chili powder and 13/4 cups of each of the cheeses until smooth. Add cooked pasta and pepper mixture and stir until combined.
Turn out mixture into prepared pan and scatter remaining 1/4 cup of each of the cheeses over top. Bake at 350° for 30 minutes, until bubbly and lightly browned.
Try chipotle chile powder for a layer of smoky flavor.
