Quoted from Creagen:
Is the entire playfield in that picture suppose to be made out of plastic? The bottom side of it sure makes it look like plastic http://www.ipdb.org/showpic.pl?id=1386&picno=11991&zoom=1. I know mines not made out of plastic.
If I were researching why some KISS playfields have color matched inserts I would probably reach out to mrbally xtheblackknightx cfh with a PM. Could do some searching on rec.games.pinball
In the Pinside game archive two of the twelve images show color matched inserts. Both were uploaded by "Guest", so that's a dead end.
There are a lot of forum topics linked to KISS https://pinside.com/pinball/archive/kiss/forum but no relevant posts use the words kiss color inserts
Games often have Engineering samples, Prototypes, and Production versions. Good luck tracking down some info. I think it's an interesting question.