Bring a large pot of salted water to a boil and cook the macaroni until al dente according to package directions.
Drain the pasta and rinse under cold water until cool; drain thoroughly to remove excess water.
While the pasta cooks, finely chop the red onion, celery, green bell pepper, red bell pepper, and parsley and place them in a large mixing bowl.
In a medium bowl, whisk together mayonnaise, sour cream (or Greek yogurt), sugar, cider vinegar, Dijon mustard, and garlic powder until smooth and the sugar dissolves.
Add the cooled, drained macaroni to the bowl with the chopped vegetables, pour the dressing over top, and toss gently to combine.
Season with salt and black pepper to taste, adjusting vinegar or sugar if desired, then refrigerate or serve chilled.
