Why do egg yolks turn blue?

Egg yolks can turn blue due to a chemical reaction between the iron in the yolk and the sulfur in the egg white. This reaction can occur when eggs are overcooked, leading to the formation of ferrous sulfide, which gives the yolk a blue or greenish tint. Additionally, certain cooking methods or prolonged exposure to heat can exacerbate this color change. It's generally harmless, but it can affect the visual appeal of the dish.