I have read at least re wagons, that paint was mixed in drums ad hoc ,hardly a scientific process, every drum would be a different paint shade.
No idea if Locos and Coaches were ever painted in any different manner?.
I doubt very much that a 100 plus years ago, that shade variation was'nt rife between each batch from various builders. After cleaning with oilrags etc, etc, how much different/similar would they actually ever been?? I doubt anyone actually ever knew even then, and were they ever bothered either ?,and now all we rely on now very old samples, for any colours for anything from that time period.
Prescision Paints are about as good as we will ever get now, no idea where their colours shades have actually come from, and from how long ago ??