I bought a bag of white potatoes and stored in humid area, covered well, and they turned green. Why would this happen?

Once potatoes are out of the grower/shipper’s ideal controlled storage (ideal humidity, temperature and no light) the shelf life begins ticking. The potatoes are washed, sorted, packed and shipped, usually within hours of coming out of the storage sheds and rushed to market. Going through the transportation, retail  (or foodservice) warehousing chain and ultimately to you, ideally the potatoes will have minimal exposure to light. However, any light exposure along the way can trigger the greening process that can progress, even if you have done your due diligence and kept them in a dark, cool place. The good news is that greening potatoes are still very useable, simply peel off the green skins (which will have a bitter flavor but is otherwise not harmful).

Most of the time potatoes will keep well under these conditions for about a week before the greening and sprouting begins, so it’s always best to use the potatoes promptly.