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This is the best indicator of a bad egg. A bad egg will have a pungent, foul odor when you break it open. The sulfurous smell will be obvious as soon as the egg is cracked (and possibly before), and the egg should be thrown out. A bad egg will have this foul smell, regardless of whether it is raw or cooked. The color of the yolk will change based on the diet of the hen who laid it, so the shade of yellow or orange nothing to do with freshness. Instead, inspect the egg white, or albumen. If it is pink, green, or iridescent, the egg has been contaminated by Pseudomonas bacteria and is not safe to eat. If you see black or green spots inside the egg, it has been contaminated by fungus and should be thrown away.  If the yolk of a hard-cooked egg has a green ring around it, this means the egg has been overcooked or was cooked in water with a high iron content. This egg is still safe to eat.  If there is a blood or meat spot on the egg, it is still safe to eat and does not mean the egg is contaminated or has gone bad. A blood spot occurs when a blood vessel ruptures when the egg is forming and has nothing to do with freshness.
Break the egg open and notice if it smells. Break the egg into a small dish and inspect the color.