It could be the diet. Especially since the petstore might not have fed them any vegetables/fruits.
I'm not absolutely sure so someone else can correct me if I'm wrong:
Maybe the extra water in the vegetables and fruits caused your birds to have more urine in their droppings. I know when birds drink a lot of water, their droppings will have more liquid/urine.
Green could be a combination of seeds and green vegetables. I believe a seed diet will make bird poop a dark green color, but maybe with the combination of seed and green veggies it is now extra green?
These birds are also in a new environment with new people, so stress can make their poop more watery too; it definitely happened to my parakeet when I moved him with me to college. His droppings went back to normal after he settled in (could take a week or two).
I would maybe try slowing down the vegetable/fruit feeding? Feed less and see if the droppings improve, or stop completely for a few days and see if there is a change?
Again I'm no expert and this is only my guess as to what might be wrong. Hopefully someone else can give more info.