Potatoes will often go green when they're not stored properly and they're exposed to light. This is due to formation of chlorophyll (which is found in all green plants), however the green colour is a useful indicator that levels of certain toxins that are harmful to humans, known as glycoalkaloids, may be increased.
Are green potatoes safe to eat?
A person should never eat a potato that is extensively green. Cooking a green potato does not render it safe. If the green portion of the potato is only on the top of the skin or in a small area, a person can usually cut out that portion and safely eat the remainder.
How much green on a potato is safe?
A recent study suggested that a 16-oz (450-gram) fully green potato is enough to make a small adult ill. Cooking does not destroy the solanine toxin, so the green parts of potatoes should be removed entirely.