Excessive exhaust heat to headers is the greatest enemy of a nice ceramic coating job.

Almost impossible to control over a period of time. A misfiring spark plug or a cam breakin can do it, among other reasons including age. My TTI's which started out very nice, dulled down to dark grey, had them recoated at Caps in Fresno,,,,ok for awhile, now grey again. Something like about 1,500 hundred degrees F will do it,,,,check it out on google.