Bring a large pot of salted water to a boil. Add macaroni (1 pound) and cook until al dente, about 9 minutes. Rinse under cold water to stop cooking then drain well.
In a medium bowl make a dressing by whisking together mayonnaise (2 cups), condensed milk (1 can), vinegar (1/2 cup), paprika (1 tablespoon), mustard (1 teaspoon), salt, and black pepper until smooth.
In a large bowl add the cooked pasta, onion, green and red bell peppers, celery (1 cup), and whisked dressing. Fold gently until well combined. Refrigerate overnight and serve.