Chop yellow bell peppers and onion and mince garlic. In a 3 to 4-quart heavy saucepan cook yellow peppers, onion, and garlic in 1 tablespoon oil, covered, over moderately low heat, stirring occasionally, until peppers are soft, about 30 minutes. Stir in cream. In a blender puree mixture in batches (use caution when blending hot liquids), transferring to a large bowl, and season with salt and pepper. Sauce may be made 2 days ahead and chilled, covered. Bring sauce to room temperature before proceeding.

Thinly slice mushrooms lengthwise and cut red and orange bell peppers into 1/4-inch wide strips. In a deep large heavy skillet cook mushrooms and peppers in remaining 2 tablespoons oil over moderately high heat, stirring occasionally, until peppers are softened and most of liquid is evaporated, about 8 minutes, and season with salt and pepper.

Preheat oven to 375 degrees. Fill a 6 to 7 quart kettle three fourths full with salted water and bring to a boil. Stir in ziti and cook until just tender, about 10 minutes. Reserve 1 cup pasta cooking water and drain ziti well in a colander. Rinse ziti and drain well. Thinly slice scallion greens. Into sauce stir ziti, mushroom mixture, scallion greens, 1 1/2 cups Parmesan, reserved cooking water, and salt and pepper to taste. Spoon mixture into a 3 quart shallow baking dish and sprinkle with remaining 1/2 cup Parmesan. Bake ziti in middle of oven until hot and pasta begins to brown, about 20 minutes.