A brief explanation of the differences and uses of the most common varieties of onions, shallots, and scallions:
Yellow (also known as brown) onions are the most common because they store well, are flavorful, and hold up well when cooked. Like most others they increase their sweetness reduce their sulfur content as they cook which develops a more complex flavor. They also brown nicely. These are good for dishes that have longer cooking times such as soups, sauces, and stews.
Red onions brighten up dishes with their color and slightly sweet flavor when used raw. They have a crisp texture. Raw they tend to leave more of an aftertaste than raw yellows. I often soak them raw in water for about 30 minutes to dial down the aftertaste. They are good for grilling, salads, sandwiches, burgers, and pickling.
White onions are very crispy with a sharp flavor when used raw. To minimize their aftertaste rinse after chopping. They add a nice crunchy texture when used as a raw ingredient. These are not sweet onions, but they develop a delicious complexity and sweetness when cooked. They are versatile because they are good both raw and cooked and are great for Mexican food, salads, and white sauces.
Shallots have a subtle flavor that doesn’t overpower other flavors like onions sometimes can. Shallots are good used in sauces, egg dishes, and salad dressings.
Sweet onions like Vidalia are very similar to yellows but with a milder sweeter flavor. The structural difference is the thicker walls of the layers. The thickness makes them good for frying but they tend to break down quickly when used in cooking. They are also a good chopped as a raw ingredient.
Green onions (also called scallions) have a mild flavor that is delicious raw or cooked. They are very versatile and add a pop of color when used as an ingredient in any dish. They make a nice garnish too. They are good for salads, stir fry, egg dishes, noodle dishes, and pretty much anywhere else you think they might belong.