Onions can be divided into two categories: spring/summer fresh onions and fall/winter storage onions.

Spring/summer Fresh Onions
Spring/summer fresh onions are available in yellow, red and white throughout their season, March through August. Fresh onions can be identified by their thin, light-colored skin. Because they have higher water content, they are typically sweeter and milder than storage onions. This higher water content also makes them more susceptible to bruising.

With its delicate taste, the spring/summer onion is an ideal choice for salads and other fresh and lightly-cooked dishes.

Fall/winter Storage Onions
Fall/winter storage onions are available August through April. Also available in yellow, red and white, storage onions have multiple layers of thick, dark, papery skin. Storage onions have an intense flavor and a higher percentage of solids.

Storage onions are the best choice for savory dishes that require longer cooking times or more flavor.

The Color of Onions
Onions come in three colors - yellow, red, and white. Approximately 87 percent of the crop is devoted to yellow onion production, with about 8 percent red onions and 5 percent white onions.

Yellow onions are full-flavored and are a reliable standby for cooking almost anything. Yellow onions turn a rich, dark brown when cooked and give French Onion Soup its tangy sweet flavor. The red onion, with its wonderful color, is a good choice for fresh uses or in grilling and char-broiling. White onions are the traditional onion used in classic Mexican cuisine. They have a golden color and sweet flavor when sautéed.

Onion Sizes
Onions range in size from less than 1 inch in diameter (creamers/boilers) to more than 4.5 inches in diameter (super colossal). The most common sizes of onions sold in the United States are the medium (2 to 3 ¼ inches in diameter) and the jumbo (3 to 3 ¾ inches in diameter).

Reference: Bring on the ONIONS