Why do we observe a greenish ring around the yolk of hard-boiled eggs?

Perfect cooking: the foolproof method

There is a simple technique, approved by all those who want perfect hard-boiled eggs, without unwanted coloring.

Always start with cold water.
Place your eggs in a saucepan, cover them with a few centimeters of cold water and heat gently.

Once boiling, stop everything.
As soon as the water starts to boil vigorously, turn off the heat. This prevents the egg from overheating.