The Train illustration is interesing for the presentation of "black" leather, "brown" leather & cord.

I wonder if anyone with better knowledge could explain the when and why the variation in use of black versus brown leather. My recollection is that brown leather is more supple and might even last longer as the process of blackening had the effect of reducing life.

If the Martinet depiction is accurate for the French, did other armies follow the same division between black leather, brown leather and cord?