I seem to recall many discussions on this over the years. One problem with a subject that old is that there were probably no usable color specs from back then, so even if you found an accurate color photo of how it looks right now, no guarantee that that is the color it was originally painted. But, since it would be hard for anyone to really argue your color, that means it also gives you more freedom to mix what looks good to your eye.
One suggestion I have is that eighteenth and nineteenth century paints tended to be less saturated than today's paints. The primary color paints from model companies (those not a specific military color, are highly saturated. I would mix up a gray of the same brightness as the orange/yellow you want to use, and keep adding the gray until it tones down the color a bit. That would also make it look a bit weathered.