“Common onions are normally available in three colours: yellow, red, and white. Yellow onions, also called brown onions, are full-flavoured and are the onions of choice for everyday use. Yellow onions turn a rich, dark brown when caramelized and give French onion soup its sweet flavour. The red onion is a good choice for fresh use when its colour livens up the dish. It is also used in grilling and char-broiling. White onions are the traditional onions that are used in classic Mexican cuisine. They have a golden colour when cooked and a particularly sweet flavour when sautéed.”
Wikipedia