Potato tubers turn green when they are exposed to sunlight during growth or storage. The green comes from the pigment chlorophyll. Potato tubers exposed to light will become green naturally as the plant seeks to harvest the light. Solanine is a toxic substance that is produced naturally in potatoes and other plants to aid in resistance of insects and animals. Solanine is present in all parts of the potato plant, including the sprouts, roots, leaves, flowers and fruits. Solanine has a bitter taste and can cause stomach symptoms such as vomiting and diarrhea if you eat enough of it. If ingested in large amounts, it can cause illness or even death in extreme cases. Because of the bitter taste, most people do not eat enough to get sick. In addition, consumers always should avoid consuming sprouts.