Hard-boiled eggs are so convenient and versatile that it’s worth keeping a few on hand at all times (they keep for a week). If you have hard-boiled eggs, cooked grains, and some sautéed vegetables in the fridge, you’ve always got a meal.
I think they’re best when cooked for 9 minutes so the yolk is just slightly undercooked and still creamy, not chalky. At 11 minutes (see the variation) the egg is still edible but dry.
If the yolk is green, it’s due to a harmless amount of sulfur in the egg, and to not cooling the egg quickly enough. To prevent or minimize it, be sure to immerse the eggs in an ice bath as soon you remove them from the simmering water.