What makes hard boiled egg yolks turn grey around the edge. And while we are all here, is it grey, or gray?

I occasionally eat a hard boiled egg as a quick “to go” snack or on a salad. It is really the only time I eat eggs so I boil up a dozen and keep them in the refrigerator for a couple of weeks until they are gone. I notice that some of them, I figure the older ones, turn grey around the edges of the yolk. They must not be rotten, because I eat them and nothing bad happens. But why does that happen. Why do they turn grey? Or do they turn gray?