The tag painting looks to be part of a much larger picture of worker discontent due to a number of factors unfolding at that time ranging from the introduction of new manufacturing processes, new car line (Camaro) introduction (and along with new Camaro RPO’s codes?) plus the consumer demands for the new Camaro that led to increased worker work load to keep up with production pace. The fact that the painting was not done on every tag shows, IMO, it was more of an employee visual aid method and possible show of union “protest” to work conditions and even a resistance to the new broadcast change which they felt was more complex especially with the older workers. Sure, the broadcast sheets began in 1966 but reading the book shows a lot happening around that time to place more demands on the production line workers that eventually led to the Jan 1967 strike. I don’t think it matters when it was done (Fisher vs. Chevy side) but more of why it was done.
As for appearing in 1968, it may have likely been a carryover practice until the workers realized that the trim tag option codes were no longer printed (to force the issue of using the broadcast sheet) and also the Firebird was slated to begin production at Norwood in 68 (page 102 explains this more clearly).
If anyone has better explanations then feel free to contribute. I’m not plugging the book but it sure does shed light on a much broader picture to see why decisions were made and why things happened the way they did.