Green skin is caused by exposure to sunlight and develops when potatoes are growing close to the soil surface or even partially above ground. The green color is due to chlorophyll, which is harmless, but exposure to light also increases the production of glycoalkaloids such as solanine. Solanine is a plant defense compound that imparts a bitter taste to the potato - ingesting too much can cause nausea and other gastrointestinal problems. if there is only a small area of green on the potato, you can discard that part, but if the greening is extensive, you probably want to toss the potato. How to prevent - mulch or mound up soil around the stems to keep the developing potatoes in the dark.