If your vodka turned dark green or even brown, the chances are its OK.
And for the amount... i would suggest you to drink the same amount you usually smoke. You said you used 1.5-2g of weed in 30ml of vodka, thus each 10ml of vodka contain 0.5-0.66g of weed in it.
So, if you usually smoke, lets say, 0.5g, then drink 10ml. If you smoke 1g, drink 20ml, etc. It must be enough. Just remember to drink it with an empty stomach (as it usually doesnt work on a full stomach). And remember also that the effects take some time to start (between 30min and 1-2 hours), so dont assume it didnt work before waiting enough time. And good luck! If it were rightly done, you surely will know it!


EDIT: 2,420th post!

__________________
"Everything we do, everything we are, rests on our personal power. If we have enough of it, one word uttered to us might be sufficient to change the course of our lives. But if we don't have enough personal power, the most magnificent piece of wisdom can be revealed to us and that revelation won't make a damn bit of difference." Don Juan Matus - Tales of Power