Re: covered hopper grays
I don’t know if this will be of any help, but here goes.
About 30 years ago I obtained permission from ACF to cut slivers from larger paint samples that are glued to the inside covers of bill of materials documents. In the mid-1990s these documents were donated to the Barriger National Railroad Library, part of the St. Louis Mercantile Library on the campus of the University of Missouri-St. Louis, where they are available for viewing.
I lined up 17 gray paint samples that pertain to 1958 cu. ft. covered hoppers on the scanner glass & scanned using an Epson V100 scanner. The ACF lot number & railroad or company reporting marks are shown.
While I believe the scanner has done a “credible" job, however, there are some hues that the scanner hasn’t picked up accurately. In particular lot 3376 SSW has a slight olive cast that I don’t see in the scan. The Lot 2963 BM sample appears to my eye as battleship gray, but the scan has a darker hue. I cannot explain the differences in the hues seen in the scan vs. actual.
There are 8 more paint samples of 1958 cu. ft. cars that I don’t have in my possession, but they are in the bills of materials (the Barriger Library calls them "Lot Books"). These include lots 2430 SHPX (same as lot 2412), 2458 L&N & 2490 NC&StL (same shade of color bordering on olive), 2597 CG, 3102 DPCX, 3143 KSMX, 3302 GM&O, 3475 RI.
A few more available paint samples include ACF triple covered hoppers lots 2833 SSW, 2841 T&P, 3193 M-I.
That these same paint samples appear vastly different (much lighter) when taken outside & viewed in natural light, especially in bright sunlight.
Perhaps this will provide at least some idea to the relative differences of light, medium, and dark gray shades as denoted in my RP CYC Vol. 27 captions. All of this, of course, doesn’t taken into account any scale affect that will cause the color used to appear darker on a model.