Accuracy in paint and lettering schemes can be an elusive thing to pin-point in time. When a railroad issues directives that changes the color and/or lettering, it takes a fair amount of time (often years) to accomplish that on a class of freight cars.
When a change is made, its usually done when a group of cars is shopped for an upgrade, heavy repairs or an overhaul. This was done when B&O changed its box car color of 'freight car brown' to bright oxide red (Floquil's zinc chromate primer was good match) in the early 1950's. B&O lettering changed too. It took the better part of that decade to redo their huge fleet of boxcars.
Similarly, UP would have repainted stock cars as they needed upgrading, overhaul or repair. For example, changing to roller bearings and replacing with new trucks. A re-paint campaign would have also been carried out, if there was no immediate need for those cars. In any event, its likely that not all stock cars got repainted with new liveries right away. It took time, (perhaps at least a year or two?) before all stock cars wore the new paint job. Both red and yellow stock cars likely worked together for a while.
I modeling, perhaps the brown car should carry some weathering and the yellow car look fresher? For a railroad, it's much more important to keep cars working and in operating condition than it is to for them all to have the same color paint job.
S. Islander