PE = Polyethylene
PET = Polyethyleneterephtalate (=Polyester)
RC = Resin Coated (here in the meaning of laminated with plastic foil; to my understanding only PE foil has been used for this.)
Thus you have prints consisting of
-) a plain paper base, with some baryta coating, (called `fibreŽ),
-) a base from paper laminated on both sides with PE-foil (called `RC-paperŽ)
-) and a base from a plain PET sheet.
PET is commonly used for film support, not for reflective materials as we think of them. It can be made translucent and semi opaque (reflective) by various means.
We think of PET as Estar, Cronar and a variety of other names.
I have color snapshots from the 1950's and 1960's on fibre-based color paper. I can tell you from personal experience, they are inferior in every way to the more modern RC color paper. The while borders are much more yellow, and the images do not have the "depth" that modern papers do. What is good for b/w is not good for color in this case. I worked in a lab in the late 1960's that printed on Kodak pre-RC color paper. It was a long process, and the emulsion was very delicate. The roll paper, processed in a big Pako brand processor was dried face up on a big heated drum before being rolled up on a take-up spool. I remember one time, the paper getting stuck to the big drum. What a disaster that was.
The process using a blix was partly made possible by RC. Paper support tends to color more from the Ferric EDTA complexes if used with FB. And it takes a long wash.
That inferiority, except for the drying, does not neccesarily need to be an RC-issue.
It also could be due to the barytage and the emulsion itself.
No, the yellowing (in prints made by Kodak) is not due to insufficient washing. The yellowing is due to the nature of the fibre base to absorb the processing chemicals and become stained by them, regardless of how much one washes. I have never seen a fibre based color print that had as nice a white border as modern RC color papers. The bleach used in color print processing is quite staining, in my opinion. something we don't find in b/w printing.
Prints were somewhat yellow after processing and some yellowed with age depending on the dye stability of the paper. This varied and improved with each generation of paper. Stable brighteners were not available at that time either.
FB paper absorbed some chemicals and the process chemistry and washes were needed to remove the chemicals from both the baryta and the paper fibres.
Just as today, you see shorter washes with RC B&W, and longer washes with FB B&W, the same is true for color, with the added problem that the chemicals create colors.