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, and reduce heat. Cover and cook for 15 to 20 minutes or until tender. Drain, mash with half the cream cheese and sour cream and all of the colby cheese.
Drain the russet potatoes; mash with the remaining sour cream and cream cheese. Add the milk, Parmesan cheese, salt and pepper; mix well.
Spread 2 2/3 cups russet potato mixture into each of 2 greased 11x7 inch baking dishes. Layer with 4 cups sweet potato mixture. Repeat layers. Spread with remaining russet potato mixture.
Bake,uncovered,at 350 for 15 minutes or until heated through. Combine topping ingredients;sprinkle over casseroles. Bake 2-3 minutes or until cheese is melted.