When potatoes are exposed to light their skins turn slightly green , this is caused by a toxic glycoalkaloid called solanine.
Solanine develops if potatoes aren’t fully covered by soil while growing, or if you leave them in the sun too long after the harvest, or aren’t stored in complete darkness.
Because solanine is slightly toxic, it’s possible to get sick if you consume a large enough helping of greened potatoes.
Peeling or cutting away green sections before cooking usually eliminates the problem, as most of the solanine is located in the potatoes skin.
Solanine, in abundance, is found in potato tubers that have been exposed to sunlight while growing.
That is why you should never eat a potato that has been exposed prior to harvest for any length of time.
The telltale sign is a greenish tinge of the potato itself.
Related: Why is my raw potato black inside?