The SP changed their colour in the early 1950s to a more of a red brown. Tru-Color makes this shade. I have used NP freight car Brown for the older colour. You can also use Star Brand SP/UP freight car brown.
Tru-Color darker colours are matched to AC&Friday paint swatches (drift panels) by historian Ed Hawkins. They need to be adjusted for indoor viewing. You add 5-10% gray or cream to adjust the colour.
Gene
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