is it true or not (green potato)?

Yes, they are poisonous. They contain a harmful chemical. You'd have to eat a lot in one sitting, but still. Read the details here: http://www.snopes.com/food/ingredient/potato.asp

Potatoes going green have begun to accumulate toxins (the potatoes way of getting everything not to eat it). Theoretically, a human can tolerate a little of those toxins but if you eat green potatoes all the time, then you will poison yourself. Cooking well will only destroy some of the toxins. So, to be safe don't eat the green parts of a potato.