Why do hard boiled eggs turn green?

You cooked it too hard and too long. The gray-green is a chemical reaction between iron in the yolk and sulfur in the white. It's hard to control the cooking of a hard-boiled egg, especially if you start out with refrigerated eggs. In most countries, they don't refrigerate eggs, and they'll last months that way. Get your eggs out of the refrigerator a day ahead if you must boil them. Avoid cooking them too long, and plunge them into cold water to stop the cooking when you're done. Eggs are delicate, and boiling water turns them rubbery as well as producing that discoloration. Or do it the smart and easy way. Just put the eggs in the oven (I use a half-sheet pan, but my neighbor uses a muffin pan) and bake them. Eggs are creamy, not rubbery, if you cook them in the oven. The perfect temperature is supposedly 154F, but my oven has a digital temperature control that has 5 degree increments, and I don't trust ovens to be perfect temperature, so I set the oven to 160F, and come back in an hour or two. Time isn't critical.I've heard of people leaving them in for 18 hours and they're still just fine.