Actually, they are. Green potatoes contain a toxic substance called solanine that nature intended as a defense against insects, but unfortunately it's also effective against humans and can cause nausea, headaches, and neurological problems if eaten in large enough quantities. Generally speaking the average adult would need to eat an entire large (16 oz) fully green potato in order to notice any symptoms, so although it is there the threat isn't that large. Solanine develops in potatoes when they are exposed to light and warm temperatures, so you can easily avoid the issue by storing your spuds in a cool dark place, and by cutting off any green spots before you eat them.