I'm a 'Primarily yellow onion' kinda guy. I don't notice a huge amount of difference ( other than color) too keep multiple varieties on hand.
During the seasons I get a long-day variety pack of onions to grow ( yellow, red and white) for variety, then another pack of only yellow ( Patterson onions) which are good for long-term storage. I find the white ones to be the strongest flavor ( raw) although all onion dull down and sweeten up a bit when cooked. I do like the contrast of colors when I make a bean salad ( preferably the red) but I usually wont go out of my way to get it, unless I have them from the garden. Leeks I primarily use for potato leek soup ( I'd write vicchysoise, but I never get the spelling right on the first try, and am too lazy to look it up
). I only use the whites and the pale green portions for the soup, but sometimes I will use the darker leaves for a stock, cause I kinda feel bad throwing half the vegetable away ( into the compost). I don't notice enough difference in shallots ( to onions) to use them regularly, but will occasionally use them. I did grow them in the garden a few times, but they are never larger than a large garlic clove, so its a pain in the but to peal a dozen, just for one recipe. But, I do use there greens in place of scallions or chives, cause they send up multiple skinny leaves ( thiner than onions but thicker than chives) all at once, so within a few weeks, I have the mother - load of 'scallions'. They also store for ever so I'll stick them in small vacant areas the garden too have a continuous flow. In general, I find most onions interchangeable when cooked ( with few exceptions). The main reason I may select different colors is when making some kind of salad, and looking for a cool contrast of colors.
When the recipe calls for minced or finely chopped, I really dont care much, but if there diced or sliced, I prefer a ' meatier' onion that has thicker layers, so they dont get lost in the dish. I also prefer onions slightly oblong , as compared to round, so when you cut off the ends prior to pealing, you cut off as little as possible ( Unless Im making a stuffed onion, in which round is a more workable shape).
In the winter, if there are no viable green ons outside to use, I'll ge a bunch of scallions and cut them down to about and inch or two to the root end. I will then put them in a glass ( so the roots are submerged ) by a window, and it will continuously produce more green leaves to use. If the soil is workable outside, I'll just thumb them into the grown and let them grow. Eventually they will go to seed, but I can still get many green leaves to use.
Last year I tried a new variety ( new to me), that I think I heard about while watching Stanley Tucci's cooking/ Italy show. They are called " Red Torpedo Tropea Onions". They store well, and are not too big, so its a nice size to use for 2 people, and not have half of an onion shrivel up, and disappear in the fridge.
