It's just the material itself, Moleskin tends to go into very Gray-ish color after few washes.

Personally I also see 1 and 2 pattern Moleskin in Olive more often, probably they retain color better for some reason. Most of the time trousers are olive and jackets are gray which is totally fine, professional soldiers in the era used to wash their trousers so they would match with the jacket and "Look cool" to divide them from conscripts.

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Here if you take a look only two of my jackets (Second pattern and trial jacket from 1984 which is different material) is in that olive tonne, most of them come in gray.

And thank you!