Hi Dave,
The harnesses originally fitted were all blue. The old harnesses in my aircraft were blue. I was told that the RAF changed to black at some point in time, the exact date was not given to me. I was also told that the webbing on the harnesses put into civil aircraft was wider than the webbing in the military version. Two inches in the civil and one and three quarter inches in the military harness. Mine were rewebbed in one and three quarter inch blue. The brown you found looks like a partial replacement as you suggest.
best regards
Nigel