I say use whatever onions you have on hand. But, typically, french onion soup is made with yellow onions, but that doesn't mean you HAVE to use that type. A sweet onion will give slightly more sweetness to the dish and will caramelize nicely while cooking on the bottom of the pot. Heck, you could even use red onions or shallots if that's all you have. Just don't use green onions or scallions. That's a totally different dish.