Soak the red onion in a bowl of ice water for 15 minutes; drain and pat dry. Meanwhile, bring a large pot of salted water to a boil. Add the macaroni and cook as the label directs. Drain, transfer to a bowl and toss with 3 tablespoons milk. Let cool.
Whisk the mayonnaise, sour cream, mustard, pickle brine, sugar, hot sauce, remaining 1 tablespoon milk and 1/2 teaspoon salt in a large bowl. Add the macaroni, red onion, bell peppers, celery, celery leaves and parsley; toss to coat. Cover and refrigerate at least 2 hours. Stir again before serving; season with salt.