Preheat the oven to 350 degrees F. Lightly butter a 9x13 casserole dish. Bring a large pot of lightly salted water to boil. Add rotini and cook until just tender. Meanwhile, melt the butter in a medium-sized pot over medium heat. Stir in flour until well incorporated and light golden in colour. Slowly whisk in the milk. Cook, whisking constantly, until mixture comes to a simmer and thickens; do not burn. Remove from heat and stir in the paprika and cheddar and mozzarella cheese; season with salt and white pepper. When the rotini is cooked, drain well and combine in a bowl with the green onions and sauce. Spoon half the mixture into the casserole. Dot the top with small spoons of ricotta cheese, and then spoon over remaining pasta. Sprinkle with parmesan cheese. Bake 25-30 minutes, until golden brown.