Place russet potatoes in a Dutch oven and cover with water. Bring to a boil. Reduce heat; cover and cook for 15–20 minutes or until tender.
Meanwhile, place sweet potatoes in a large saucepan; cover with water. Bring to a boil. Reduce heat; cover and cook for 15–20 minutes or until tender. Drain; mash with half of the cream cheese and sour cream and all of the Colby cheese.
Drain russet potatoes; mash with the remaining cream cheese and sour cream. Add the milk, Parmesan cheese, salt and pepper; mix well.
Spread 22⁄3 cups russet potato mixture into each of two greased 11-by-7-by-2-inch baking dishes. Layer with 4 cups sweet potato mixture. Repeat layers. Spread with remaining russet potato mixture.
Bake, uncovered, at 350 degrees for 15 minutes or until heated through. Combine topping ingredients; sprinkle over casseroles. Bake 2–3 minutes longer or until cheese is melted.