Since color photography wasn’t available during that period, any photos you find will always be B&W. From what I understand, both sides used existing railroads to transport troops and equipment so the color schemes you could choose from would probably match those railroad’s colors of the area.

I might suggest that you rent or purchase an old Walt Disney film called the ‘Great Locomotive Chase’ to get some ideas about color schemes. They may not be 100% authentic but it should give you a starting point.

Hope this helps,

Geof from the US