Make the white sauce using the butter/margarine, flour, milk, salt and pepper (Video here). Stir in the chopped onions, chopped green peppers and taco seasoning once the sauce is bubbling and slightly thickened.
Cook pasta as directed. Drain and add to large mixing bowl. Mix in the leftover cooked chicken and cooked beans.
Pour the creamy taco sauce over the pasta, chicken and beans. Toss.