There are about as many opinions about this as there are people!
I have read in the PRRT&HS magazine (Pennsy historical society) that the color became darker over the years. This implies that the darker brownish is probably better on the shadow keystone cars and the lighter reddish on the circle keystone cars.
However, the paints especially earlier ones with natural pigments were strongly affected by weather(versus newer synthetic pigments), both sun and heat and cold and ice and rain and snow. So the colors changed. also, the where the car has been, ie service or whatever will affect the color. It will pickup local dust of various colors and weather based on the weather. So in my opinion many many shades are possible and prototypical.
Next the light we view the car in has an affect. Seeing it bright sunlight outdoors it will look one way, and seeing it in your basement under whatever kind/color of lighting it will look different.
And people who remember exactly what it looks like, are remembering stuff from the mid 1950s or earlier. My memory of those times is OK, but I am sure I can't identify finer shades of color on RR car or automobiles. So that as a source is suspect in my opinion.
There have been color drift cards offered by the historical society for various colors on the RR. these would apply to what was ordered, and what it looked like right out of the shops - maybe. All the fading and weathering would begin to apply immediately, especially on older circle keystone cars. I have worked in industry and based on my experience the shop forces used whatever paint was delivered. They got paint in 55 gallon drums and they did not compare it to a drift card when they opened it, they just used it. So if a batch came in a different shade, the shop most likely did not notice or care. Their purposed was to clean and seal the equipment against rust, so they used what came in.
there are few color photos from the 50s on back to earlier times. These are interesting but again, they are over 70 years old and even Kodachrome fades some in that time. Ecktochrome and the Anscho films tend to fade a lot over that time frame. Whatever colors show up on the slides are a good guide, but not in my opinion a final definition.
To sum up, I think there were many shades of Pennsy Freight Car Red floating around at any given time on the prototype and can be on the model. As an avid Pennsy modeler, I have favorites (and the lionel dark brown is not one) but I accept a wide range of shades before I call foul!