How do you tell if a hard-boiled egg is bad?

I fixed some on April 7 and dyed them the next day. I went to eat one
yesterday and part of the yolk was gray. About half the yolk was
yellow and the other half was completely gray. I've read that the gray
is a sign that it was overcooked and it's harmless to eat. However,
I've never seen one this gray before and I've been fixing hard-boiled
eggs the same way for years. None of the other eggs I fixed were this
gray. Also, it smelled really bad. It didn't smell rotten, just bad.
I figured if it smelled this bad, it probably tastes bad too. So I
didn't eat it.

Would it have been ok to eat?