Spanish omelette

[PositronicRay]

Last time I made one of these it went an unappetising greyish colour when cooled. How do I keep it a nice yellow eggy colour?

[21TonyK]

Normally means the egg is overcooked, like if you over boil a hard boiled egg you get a black ring around the yolk from the iron in it.

[PositronicRay]

Ah ok, I do cook these things for ages.