When purple beans are cooked and they turn green I understand that is because the anthocyanins are only purple in an acidic environment. So, when the beans turn green after cooking does that mean there is no anthocyanin remaining?
The anthocyanins are still there; they've just changed color. They're an indicator, like litmus, except that they range in color from red to purple to blue to green to yellow, depending on how acidic or alkaline the liquid is that they're in, instead of just pink or blue.
Red cabbage will do the same thing.