Original canvas color question

I found an old military canvas in my pile of parts. It is OD green, but appears to have been either stained or painted black on the outside, the inside is still green. Does anyone know how this was done? I know for sure this top is military surplus (or actually used military, not a civilian paint job). It is flexible and doesn't really look like paint, but something that has stained it black. Any info would be appreciated...