Preheat the oven to 350 degrees F. Bring a large pot of lightly salted water to a boil.
Meanwhile, make sauce by melting the 1/4 cup butter in medium pot set over medium heat. Add the bell pepper, celery, zucchini and garlic. Cook the vegetables 4 minutes, and then stir in the flour and cook 2 minutes more. Slowly, while stirring constantly, mix in the milk. Bring the mixture to a simmer, stirring occasionally so it does not scorch on the bottom. Add the pasta to the boiling water, cook until tender, about 8 to 10 minutes, and then drain well.
When the sauce has come to a simmer, remove from the heat and mix in 2 cups of the cheese, green onions, salt, white pepper and paprika. Mix the pasta into the sauce, and then spoon into a lightly buttered 9" x 13" casserole. Sprinkle with the remaining cheese. Bake for 30 minutes, or until bubbly and delicious.