Seems obvious to me that the red accents were to make the feature area "pop", and somehow the bean-counters and assembly plants signed off on it.
The musclecar market was very competitive: that's why little things were done to differentiate & hype interest.
(Side note: bear in mind for all this stuff that cars were usually traded in at 3 years; also, automakers were just trying to get through the warranty period.)
Yes, red fender liners were an Olds 442 thing for a while; I had a '68 Barracuda 340-S that also had them, very rare. They did look a bit odd on a light green car.