Put the white cabbage, red cabbage and apple together in a large bowl. In another bowl, mix the mayonnaise, sour cream, vinegar, mustard, celery seeds or celery salt, salt (a little less if celery salt is used), pepper and sugar.
Add to the cabbage and mix well. Cover and put in the fridge for 1 hour. Season to taste again before serving and sprinkle with chopped parsley.