Cook pasta according to package directions. Drain well. Rinse with cold water and drain again.
For dressing, in a large bowl stir together yogurt, mayonnaise, milk, dill, parsley, lemon peel, lemon juice, and pepper.
Stir the pasta, cucumber, tomatoes, sweet pepper, and green onions into the dressing. Toss gently to coat. Cover and chill in the refrigerator for 3 to 6 hours before serving. Fold in olives just before serving.