Most likely the manufacturer Athearn used did different runs of the two kinds of boxcars at different times with slightly different paint colors. Keep in mind paint fades in real life, so on a real railroad you could see color differences from one car to another, even if they started out exactly the same color.
Keep in mind too that Athearn may have been using the same brand of paint, but it may have been “reformulated” to meet recent environmental laws or regulations. I use Tamiya paint a lot, and I know they’ve been changing their paints recently. Their old “olive drab” looked pretty close to me to a slightly lighter Pullman green so I used that on some passenger cars, but the more recent “olive drab” I’ve bought is a touch lighter than the earlier ones…but then green and blue paint fade easily in the real world so it ends up looking OK when running together.
The shade of ‘box car red’ can vary quite a bit. Most of this paint was made from clay for pigment and things like linseed oil for the transport medium. These paint are inexpensive and do fade quite fast. After WWII and around the early 1950’s, standard formula paints started to be manufactured that were consistent in color and did not fade as fast. You will note that SOO & NP boxcars from pre-1950 usually are a lighter shade of ‘box car red’ that the same cars painted in the mid 50’s - more so that just fading of the colors.
The AT&SF did have a specific ‘mineral brown’ that they used on cars, but I suspect the later cars had paint that did not fade as fast. Various shades in your car fleet add a more prototypical air to the scene.
Santa Fe boxcars were usually either Mineral Brown or Indian Red. There are other paint colors, but they don’t really apply to your modeling era.
Unfortunately many HO manufacturers Mineral Brown color has too much red in it. Mineral brown should be a chocolate brown color. Not a box car red.
Good renditions of Santa Fe Mineral brown can be seen on Kadee PS-1 ATSF box car models. And Intermountain’s caboose line.
Later on, perhaps as early as 1959, (I’d have to double check that) Santa Fe adopted a bright red color called “Indian Red”. The cabooses that were rebuilt into the Ce series also received Indian Red.
Here is what Santa Fe Indian Red looks like in its earliest application:
Before artificial paints came along, paint had to be colored using natural items like plants and minerals, so the cost of the paint would be largely based on how rare or common the ingredients were for the color. The red paint color called “boxcar red”, “oxide red” etc. was dirt cheap because it was made from…well, dirt. Technically, red clay containing iron ore, which is very common.
Naturally, different areas had different amounts of iron and other minerals in their dirt / clay, so the paint would be slightly different - similar to the way different bricks were different colors depending on where they were made. It was not only a cheap paint but wore very well - but all paint will fade over time.
BTW that’s why barns are often painted red. They were originally a natural “oxide” red, but when artificial paint came along farmers still asked for “red paint” but got a brighter caboose / fire engine red compared to the old clay-based red.
Thanks very much for the many informations and tips
Since i started to collect the ATSF rolling stock i have found a variety of shades of the brown colour used by the SF and i needed to ask this question in case this variety had a significance in terms of time period or historical changes of eras so your explanations were much appreciared