They are hot, and are the most likely to make you cry while you are cutting them. Spanish onions are a large yellow onion that is milder and sweeter than most storage onions. When sweet onions are not available, Spanish onions can be substituted. Red onions are sharp and pungent, but also with a bit of sweetness to them. Their color makes them a beautiful addition to salads, but when cooked their color fades unless lemon juice, wine or vinegar is added.
White onions are hot with a sharper flavor than yellows.
They often are used in Mexican cuisine. Pearl onions are small white, yellow or red onions, about an inch in diameter and mild in flavor. Boiling onions are usually white and about the size of a golf ball. Fairly pungent, they are often simmered whole in stews. Whatever the variety, onions contain vitamin C, calcium, potassium and dietary fiber.