In a large bowl, toss together the green, red, and yellow peppers with the tomatoes, cucumber, and onion.
In a medium bowl, whisk together the vinegar and olive oil. Season with salt, pepper, and sugar. Whisk in oregano, basil, and marjoram. Continue whisking until ingredients are well blended and the oil and vinegar are emulsified.
Pour dressing over vegetables, and toss gently with a wooden spoon. Cover, and marinate at room temperature for 1 to 2 hours, stirring frequently. Refrigerate until 1 hour before serving; then remove from the refrigerator, and let sit at room temperature. Garnish with chopped parsley.