OK, I don't think You will find colour photographs, only graphic prints and these were made for instruction and not by specialists - so I would go with common sense:
everything of wood is bound to be painted in the same colour, and everything of iron would be blackened, since there wasn't anything else to prevent corrosion of steel - ferric oxide
on campaign the black layer would get worn off, so a bit of silvery/metallic should be OK, and very little rust
as soon as the piece would look too weathered, it would be repainted black - military tends to maintain their stuff in very good shape
some parts could be of brass, especially moving ones - brass is still best and cheapest material when it comes to that
check the naval battle sequences of the Kolchak movie for this - gives a very good impression
this is my approach
as to Your pictures, I would go for black with the parts You mentioned, as well as the axle
the wheels are not detailed enough, so I would use black for the hub and greyish silver for the tires, maybe a bit rusted
brass for the small parts on the breech
the model is not very detailed, so no need to spend too much time on it