In a small mixing bowl, combine buttermilk, mayonnaise, and yogurt. Use a wire whisk or fork to stir in the salad dressing mix. Set aside. In a large saucepan, cook the potatoes in enough boiling salted water to cover; drain. In a large mixing bowl combine cubed potatoes, Cottage cheese, celery, Colby cheese, radishes, green onion, salt, and pepper. Toss lightly to mix. Gently fold in salad dressing; cover and chill. At serving time, garnish with hard-cooked eggs.