Cook pasta according to package directions. Meanwhile, place oil, catsup, mayonnaise, vinegar, sugar, paprika, salt, and pepper in a screw-top jar. Cover and shake until smooth. Set aside.
Drain the cooked pasta in a large colander and rinse under cold water until cool; set aside. Snip off the root ends from the green onions and about 2 inches of the green tops; leave some of the green tops attached. Snip trimmed onions crosswise into thin slices.
In a large bowl gently toss together cooked pasta, kidney beans, celery, red pepper, and green onions. Pour on dressing. Toss until pasta is well coated. Makes 8 to 10 servings.