An onion is a biannual, which means it has two lives. In its first life it grows from seed to set, and in its second life it produces a larger bulb. Sets are usually quite pungent and only good storage types can be kept for next year. A transplant is much less likely to bolt or go to seed than a set. Almost 70 percent of all onions are yellows, and there is a good reason for that.
Yellow onions tend to be easier to grow than whites or reds.
They generally have a tougher skin and heavier foliage. They also are more susceptible to sunburning. It is difficult to make reds achieve the desired size. This is generally why reds and whites cost more in your supermarket. A hybrid is a cross between two different cultivars.