You havent advanced far enough in Leafgreen. It wont be able to trade with any color except other Leafgreen and Firered until you get the National Pokedex and fix the Network Machine on One Island. You'll have to beat the Elite Four In Leafgreen before you can fix the Network Machine, then you'll have to travel to islands 1-6 to get the items needed to fix the machine (this answer is brief and is leaving out lots of details, look up leafgreen to get a more complete answer). You also need the National Dex in Green, to get it, capture 60 different Pokemon, and beat the Elite Four. The Sevii Island quest in Leafgreen isnt too hard, just annoying. Good luck! Emerald MIGHT be able to trade with Leafgreen after green has done the necessary busywork, but if it doesnt, beat the Elite Four in Emerald to get the National Dex and unlimited trading between all games. In the meantime, Emerald can always trade with a completed Colosseum, and Leafgreen can trade with Colosseum after the Network Machine is fixed. Good luck!