When an egg has been cooked for too long, a thin grey-green ring sometimes appears around the yolk. Its position at the boundary between white and yolk is not accidental. Heating releases hydrogen sulphide from sulphur-containing components in the white. It diffuses inwards and reacts with iron at the yolk surface, producing grey-green ferrous sulphide.
The reaction is not completed the instant a single temperature is reached. More heat and more time give molecules greater opportunity to move and react, making the ring more noticeable. The egg also remains hot after it leaves the stove, so residual heat can keep the process going. Prompt cooling therefore shortens the time available for the reaction, not merely the time before the egg can be handled.
The colour usually does not mean that the egg has spoiled, nor has pigment leaked in from the saucepan. It mainly indicates excessive cooking time or temperature, although iron-rich cooking water can intensify the effect. Gentle cooking followed by rapid cooling in cold water reduces the ring. This small kitchen detail demonstrates a basic condition of chemistry: substances must meet, while time and temperature determine how many opportunities they have to react and change.
https://ask.fsis.usda.gov/article/What-causes-a-green-ring-around-the-yolk-of-a-hard-cooked-egg
https://food.unl.edu/how-avoid-green-ring-hard-boiled-egg-yolks/
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