That was a very good analysis of the issue, and the photo showing the various mineral reds was wonderful.   

A friend and I sample paint where ever we find it.  If possible I retain a physical sample.  We also do pantone matches, then create color cards as a way of presenting the information.  An example can be found at http://www.pacificng.com/pdf/web/viewer.html?file=http://www.pacificng.com/roads/nv/cc/pdf/CC-Freight-Colors-1881-1900.pdf

None of the systems are perfect, but the photo of the variations on mineral reds suggests a solution for modelers... the cars should demonstrate the range of different colors found.  That with weathering is what gives the cars a realistic appearance.

Randy Hees