Hi Julian,
I don't think that it's supposed to have the yellow tint. After seeing such a high percentage of flawed NOS parts, I've come up with a theory that many non-mechanical factory replacement parts are ones that failed quality control inspection and deemed not good enough to go on a brand new car. They were considered acceptable to go on a car that's been in service for some time, likely to be installed by a body shop after the car was in an accident, and probably never closely inspected or seen by the owner of the car.
My guess is that there was an issue with the plating process that caused the yellow tint. Once it was noticed, the yellowish housings were separated from the rest and destined to become replacement parts. Once the housing is installed in the car, I doubt that the yellow tint will be noticeable.
I have a good number of NOS body parts in my collection, and just about every part that's plated has some kind of significant flaw. Two common issues are pitting on pot metal parts, and areas of very thin plating on bumpers.
Jeff