Place potatoes in medium sauce pot add water to cover 1 inch above potatoes. Bring to a boil, slower and simmer. Cook until tender approximately 7 minutes. Optional, add 1/2 tablespoon of salt to the water.
Drain potatoes, place in large bowl and chill for 30 minutes in refrigerator.
Combine yogurt, mayonnaise, and mustard in a large bowl.
Add onion, celery, red pepper, parsley, salt and pepper.
Stir in chilled potatoes, refrigerate until ready to serve.