Your example is not correct. That's the whole point, that the brown shade was NEVER used in the Soviet uniform of this period.
Regarding the painting of Soviet military equipment - it had the same factory color only until the first major repair. Subsequently, it was painted with the paint that the soldiers themselves (who made repairs) hand-made on the spot. To do this, they were supplied with pigments and a binder from the warehouses, but the shade was already very diverse - this was not given significant significance.