Bring a large pot of lightly salted water to a boil. Cook the tri-color pasta at a boil, stirring occasionally, until cooked through yet firm to the bite, 8 to 10 minutes. Rinse pasta under cold water until cool; drain.
Whisk mayonnaise, milk, vinegar, sugar, salt, and black pepper together in a large bowl until smooth. Add pasta, imitation crabmeat, peas, carrots, celery, green pepper, and onion; mix until evenly coated. Cover the bowl with plastic wrap and refrigerate to let flavors combine, at least 5 hours.
