Grey potatoes are still safe to eat.
When potatoes turn grey they turn grey due to oxidation.
The oxidation of the potato does not make the potato unsafe to eat but it may not taste as good.
The process that causes potatoes to turn grey, which is called oxidation, happens because potatoes are a naturally starchy vegetable.
And when exposed to oxygen, starches turn gray, brown, or even black.
An oxidized potato is completely safe to eat, the process doesn't affect the flavor or texture of the vegetable.
After peeled or grated potatoes are exposed to air they will begin to turn gray or brown.
This appearance can be off-putting, but the good news is that the quality of the potato isn't compromised.
This is a harmless natural reaction and it is perfectly fine to cook and eat potatoes even if they have turned gray.
After you remove potatoes from the cooking water, ferri-chlorogenic acid slowly mixes with oxygen, creating a color that can range from gray to black to purple to bluish.
So potatoes that seem fine when you put your dinner leftovers in the fridge may not show off their smokey new look until morning.
When potatoes go bad they will become mushy, have mold or smell bad.
If the potatoes are actually bad then they should be thrown out.