Red onion vs Spanish onion, what is the difference between the two. Its is said that the red is sweet, but I don't find that true. I find the Walla Walla onion and Vidalia are sweet. The red to me is strong onion flavor and I notice it goes sour if you cut them ahead.--Steve
Actually, all of the onions you mention are varieties from within the same plant family, Allium cepa. Within that family are red onions, Spanish or yellow onions, Bermudas, as well as the sweet Maui, Walla Walla, Vidalia types. According to Harold McGee in On Food and Cooking: The Science and Lore of the Kitchen , sweet onions are actually yellow onions that are grown in low sulfur soil. The lower sulfur content means that fewer of the astringent compounds onions are notorious for are formed.
Red onions are the same plant as yellow, except that they have anthocyanins in the surface layers of each ring. These chemicals, which give the onions their red color, appear not just there, but in other fruits and vegetables, such as eggplant, blueberries, cherries, and even the unusually colored purple corn. Since the red color is water soluble, it is quickly diluted in cooking. Also, anthocyanins are affected by acidity so the color may change or turn dull, depending on what other ingredients are used. Besides color, and perhaps slight variations in taste, the red onion is essentially the same thing as the Spanish. It is not a sweet onion in the same sense as those listed above.
Within Allium cepa, there are numerous varieties onions, each bred for different properties, such as color, size, flavor, growing season, hardiness, storage life, etc.
As to the question of going sour if they are cut ahead of time, once chopped the flavor of any onion will change with both time and exposure to oxygen, due to complex chemical and enzymatic reactions. This is likely the effect you are noticing.