The color of an onion determines its flavor, but for the most part, when cooking, any onion will do, according to Linda and Fred Griffith in their book, ''Onions, Onions, Onions." But for those with a more discerning palate, here are some guidelines:

• Yellow storage onions. Common cooking onion, sold in red net bags, is hot and makes you cry. This workhorse is good in any heated dish or one where subtlety is not an issue.

• White (storage) onions. Hot with a slightly cleaner flavor than yellows. Commonly used in Mexican cuisine. Sold individually or in blue net bags.

• Spanish onions. Larger, less hot, more sweet, often referred to incorrectly as Bermudas. Substitute for sweet onions out of season. Sold individually.

• Red onions. Sharp, sweet flavor. Uncooked, these can be added to any salad. When cooked, they lose some of their flavor and turn a greenish hue. Sold individually.

• Boiling onions. Small, yellow or white storage onions, about 2 inches in diameter. Hot before cooking, they are best left whole, boiled or simmered in stews. Sold in bulk.

• Pearl onions. About 1 inch in diameter, these sweet onions are good marinated or pickled. Boiling onions easily can be substituted for them. Sold by the pint.

• Green, or creaming, onions. Fresh, young bulb onions, picked before maturity. Good raw and cooked, they make an excellent substitute for red and/or sweet onions in salads. Sold in bulk.

• Specialty sweet onions. Low in heat, high in sweetness. Often sold as Maui, Vidalia, Walla Walla or other regional name. Smaller ones are hotter than the larger ones. Excellent raw, delicate when cooked, they caramelize nicely. Great for onion rings. Sold individually.

• Leeks. Garden-variety leeks often are very hot, coarse and chewy. When cooked, they develop an oniony flavor; when sauteed, they become buttery in texture. A yellow storage onion can be substituted, but the flavor won't be the same. Sold individually.

• Scallions. The white bulb is mild and good cooked and raw. The chopped greens make a handsome garnish. Sold by the bunch.

• Shallots. Like tender, delicate onions, they make a great base for sauces and splendid addition to omelets. Perfect for braising or roasting, but don't eat raw. Often sold by the pound.