Place eggs in a single layer in a saucepan and add enough tap water to cover at least 1 inch above the eggs. A tablespoon of vinegar can be added to allow better dye coverage after cooking.
Cover the pan and quickly bring just to boiling. Turn off the heat and if necessary, remove the pan from the burner to prevent further boiling. Let eggs stand, covered, in the hot water for 15 minutes.
Immediately run cold water over eggs or place them in ice water until completely cooled.
Refrigerate hard-cooked eggs in their cartons if you won’t be coloring them right after cooking and cooling. Refrigerate them again right after you dye them and after you display them.
Color only uncracked eggs. If you want to eat your dyed eggs later, use food coloring or specially made food-grade egg dyes dissolved in water that is warmer than the eggs. If any eggs crack during dyeing or while on display, discard them along with any eggs that have been out of refrigeration for more than 2 hours. If you keep hard-cooked eggs out of refrigeration for many hours or several days for a decoration or for hiding, cook extra eggs for eating. Either discard the eggs that have been left out or use them only for display.
If you hide eggs, consider hiding places carefully. Avoid areas where the eggs might come into contact with dirt, pets, wild animals, birds, reptiles, insects or lawn chemicals. Refrigerate the hidden eggs again after they’ve been found.
Salmonella or other harmful bacteria may be in the eggs, therefore, the FDA recommends cooking eggs until both the yolk and the white are firm, not runny. Because the shell of an egg is very porous, it will let bacteria to penetrate. Wash your hands thouroughly each time you handle an egg.
Most commercial egg producers lightly coat their eggs with a thin spray coating of mineral oil to close the pores against contamination. Cooking the egg in the shell, however, removes that barrier and the eggs are again prone to contamination.