Why rice turned pink overnight after cooking it
Obviously, the rice had reacted to some outside influence that shouldn’t be there. There could have been a fungus or a bacteria (such as ergot, but probably not ergot) on the rice.
Also, how good is the quality of the water you cooked it in? Rice is essentially a sponge in that it’s a dry, hard substance that softens and cooks by absorbing water. So the water is just as important as the rice. Even the purest, highest quality, organic rice is going to be nasty if you cook it with water you got from the Ganges River on laundry day.
It could also be the result of a chemical or pesticide on the rice. You should always thoroughly rinse any rice you buy that isn’t certified organic.
It could also be that you cooked or stored the rice in a reactive vessel. For example, you never cook tomatoes in an unlined copper pot. Or a non-anodized aluminum pot. Or a cast iron pot. You may have bought rice that was treated in such a way that it had a negative reaction to a reactive metal pot or bowl.
If you left the uneaten rice out overnight, this could also allow fungal or bacterial agents to get into it and turn it pink or red.
In any event, do not eat the pink rice!
Also, you should contact your local board of health and ask if the vendor you purchased the rice from is being investigated, or has been investigated previously. Or, maybe just start buying rice from a more reputable vendor.
How to cook rice
You should feel lucky that inedible leftover rice is your only problem. In recent years some third-world Asian countries have had outbreaks of people being sold rice that is plastic, or is a mixture of real rice and plastic rice. The cooked plastic rice made some people terribly ill
