With leeks, you want to use the white part and the pale green part -- there's usually about an inch of pale green between the white and the dark green.

The dark green is really good for making vegetable stock; you throw it in a pot with the ends of the onions, the potato peelings, the apple core, ends of carrots, squash innards, etc., add about 6 cups of water, and cook for 20-30 minutes. Then put a colander over the pot where you have the soup vegetables and just strain it right in. Good times.