I used to know what all the colors and all meant but I have had these plugs in for a while now, about a year if not more. Well, upon inspection, they are very very dark in color, and the porcelin part was very very red, like a mars red. What does it mean ? They were very similar, none of them looked darker or lighter than the other by any considerable amount. But a few of them the porcelin was more red than others and a few were just black. Thanks for any help.