It seems like it would be tough to get this to be worth the 6 it costs. You can't count on the opponent discarding anything good unless there's strong trashing (a copper does nothing for you and hurts them very little), which means it's up to you to supply the discard power. Outside of peddlers, you're usually going to have to be discarding something like a gold or a 5 in order to make the gain part of this work. That makes the rest of your turn much weaker. Even if you're drawing your whole deck (or a lot of it), that gold is still valuable to have (unless you can pick it up again).
If your opponent has done significant trashing and gotten rid of coppers, this might really shine. However, the trashing of it when you gain a green is a big downside, making it only worthwhile to gain province or colony with it, and since you're losing it when used, a gold might be more appealing. If this was put at 5 cost, it might be more viable, but I have a feeling it would be a card you'd only want to pick up in certain instances and wouldn't crack the top tier of 5s.
Also, with more players it gets stronger, but you still can't count on your opponents doing too much other than copper discarding unless you get lucky, so it's kind of a fancy cutpurse that doesn't give you the +2.
tl;dr: interesting card idea, maybe priced a little high and not always useful for each game. The trashing part is a big blow and you might consider leaving that out all together.