Boil water in large pot with salt.
Cook macaroni according to package directions until tender.
Drain pasta and rinse with cold water.
Let pasta cool completely in colander.
Mix mayonnaise, mustard, and vinegar in bowl.
Add garlic powder, salt, and pepper.
Combine pasta with dressing in large bowl.
Add bell peppers, onion, and celery.
Toss everything together until well coated.
Chill in refrigerator for two hours.
Garnish with fresh parsley before serving.