zinc phosphate on rally center caps?

Those look great but mine is no where near that dark.I wander if there was a difference in caps or years.Mine were on my 1970 dart but I can't say for certain it came from the factory that way.Also would the paint need to be heat resistant that being said I have some vinyl lettering on my calibers on my 06 charger for the last 7 years and they have not peels or changed in any way.