Yellow onions are most common. They store well.
White onions are usually sharper in flavor.
Red onions are somewhat mild in flavor. Their red skin adds color to dishes.
Sweet onions have a mild flavor even when raw. They don't store as long.
Green onions (scallions) are mild in flavor. Both the bulb and the green tops are eaten.
Chives are thin green stems with a mild onion flavor. Use them fresh or dried.
Leeks have a mild onion flavor and can be used raw or cooked.