Most underhood plastic (base) resins are common low cost thermoplastics such as Nylon, HDPE, LDPE, and Styrene based materials such as ABS, all have different characteristics (or better stated, "blends" of characteristics) engineered into them for the applications they were chosen for. The problem is that none of todays modern plastics are exactly like plastics (even identified by the same names) used 30 years ago, there are many new additives in todays blends of even the most basic polymers to add various features such as UV inhibtors that did not exist back then. Due to these changes over the years and the better refinement of the resins used in modern times the base resin colors have changed as well.

Having worked in the plastics industry for the past 20 years I can assure you that not all Nylons,PP,PE,LDPE,HDPE,or other typically natural/white/clear materials are "pure", MOST of them have a shade of color ranging from bluish to yellowish, and it can vary from one lot to the next depending on many things in how the raw pallets were extruded.

Back to my original statement, many original plastics used on Mopars were "yellowish" when brand new. To what degree is the most "correct" well that's a bit too subjective to comment on much further.