Make-ahead and mayo-free, this potluck-friendly salad combines raw green beans, which are softened by the vinaigrette, with creamy white and pink beans and wilt-proof parsley.
Soak onion in cold water 10 minutes; drain. Meanwhile, in a large bowl, whisk together vinegar, oil, oregano, and 2 teaspoons salt. Add white, pink, and green beans, celery, onion, peppers, and parsley, stirring to evenly coat. Season with salt and pepper and serve, or cover and refrigerate up to 2 days (return to room temperature before serving).