Answer - When potatoes are exposed to light, metabolic activity in the skin increases as it prepares to send out shoots. As a result, chlorophyll is formed, which provides the green color. Another product formed is solanine, which is a glycoalkaloid toxin. It has a bitter taste and can irritate the gastrointestinal tract. If enough is eaten it could cause symptoms such as vomiting and diarrhea.
http://extension.psu.edu/food/preservation/faq/green-potatoes
Lately, bright green potatoes seem to be a lot more common than I ever remember. The Russet, displayed loose or in a bag can turn green in a matter of days when customer sales are slow or a store is not rotating the display to get rid of potatoes with soft spots, bruises, ugly shapes and greening.
https://idahopotato.com/dr-potato/can-i-eat-potatoes-that-are-green
If just a light green, then trim off the green skin or flesh and boil, bake or prepare as usual. However, with what I have been seeing in retail grocery stores lately on display, I would caution you to not automatically pick up potatoes for purchase that have turned very green.
https://idahopotato.com/dr-potato/can-i-eat-potatoes-that-are-green
The best way to avoid ingesting solanine is to discard the green potato or for small spots completely cut away the green skin and section around it. Additionally if the potatoes have sprouted they too should be thrown out. Furthermore bitter tasting potatoes should not be consumed.
http://www.spiciefoodie.com/2012/12/12/are-green-potatoes-safe-plus-a-winner-fun-gifts/
It turns out that throwing out the green potatoes was a good idea because they are poisonous. The green color is due to high levels of chlorophyll which in this case means an increased amount of Solanine. Solanine is a glycoalkaloid (poison found in nightshade family) toxin.
http://www.spiciefoodie.com/2012/12/12/are-green-potatoes-safe-plus-a-winner-fun-gifts/
Still keep an eye out for it and avoid. In the photo above you can see the difference in color. The top potato is perfectly fine, the middle has a spot and I cut off the bottom halve before using. The bottom potato us green and it went right into the trash. After all the research I did I am sure glad it went straight into the garbage.
http://www.spiciefoodie.com/2012/12/12/are-green-potatoes-safe-plus-a-winner-fun-gifts/
But, in potato tubers, it is like the canary in the mine shaft.. The green indicates an increase in the presence of glycoalkaloids, especially, in potato, the substance solanine. When the potato greens, solanine increases to potentially dangerous levels. Hence it is advised not to eat the green parts of the potato. Commercial varieties of potatoes are screened for solanine levels, and most have a solanine content of less than 0.2mg/g. However potatoes that have been exposed to light and started to green can show concentrations of 1mg/g or more.
http://www.food-info.net/uk/qa/qa-fp96.htm
If more extensive greening occurs, throw the potato out. The tendency for potatoes to turn green and form solanine varies among varieties. To prevent it from happening, inspect potatoes at the store before purchasing and store them at home in a cool dark place with good air circulation.
http://extension.psu.edu/food/preservation/faq/green-potatoes
Typically if the potato has been exposed to light (sun light, fluorescent light, spots, etc.) it will convert the skin to Solanine, which if ingested in large quantities can be toxic. But, you’d have to eat much more than a single potato to feel ill.
https://idahopotato.com/dr-potato/can-i-eat-potatoes-that-are-green
Are green potatoes safe to eat ? Exposure of potato tubers to light either in the field, in storage, on the store shelf, or at home, will induce the formation of a green pigmentation on the surface of the potato. This is called greening and indicates the formation of chlorophyll.
http://www.food-info.net/uk/qa/qa-fp96.htm
is it safe to eat a green potato
No, it is not safe to eat a green potato.