They turn green when they are exposed to light. That's why one is encouraged to keep piling soil up on the plant as it grows.....to prevent any kind of light from affecting the tubers.

While the green is due to chlorophyl, it is also associated with a concentration of solanin, a natural component of potatoes but one that creates a bitter taste. It is best to peel or cut away any parts that are green before eating. If consumed in excess, solanin can cause stomach upsets.

Simply, did you notice if the tops of the potatoes were at the surface of the dirt after the plants died back? The sunlight hitting the potato skin made them green on top.