The blue-green color of purple potato porridge is actually a normal phenomenon.

Purple potatoes contain a lot of anthocyanins, purple potato soup becomes green, because anthocyanins are an unstable water-soluble pigment, because the degree of acidity and alkalinity and change color, that is to say, when the anthocyanins encountered in acidity will become red; encountered in alkaline strong will become green, alkaline weaker will become blue; neutral will remain the original purple. This phenomenon is the reason for the water, because different regions of the tap water acidity and alkalinity is different, so it will also affect the simmering out of the color of purple potato soup.

Expanded information:

Purple sweet potatoes are colored because of the proanthocyanidins, which are naturally occurring. This kind of natural pigment in the neutralized natural environment is blue-purple, acidic and alkaline natural environment is dark purple, alkaline natural environment is dark blue.

Generally boiled purple potato porridge such as water or other reasons is very easy to become dark blue or even with bright green, if you want to change the color, just need to add a little white rice vinegar or lemonade such as acidic and alkaline liquids can be changed back to the blue-purple.

People's Daily News - Why is the soup made from purple potatoes green?