Chris, I own both brown and green British Denim BD and my belief is that the brown came first, followed by the green; although at some point during manufacture you get both being produced side by side. Brown denims were issued in the UK in the 1930s for work wear and I suspect that this lead to the brown demin BDs. Some of the brown denims were almost pink, although I do not own any in this shade.
I have no idea what would have caused the change from brown to green and I have no idea if the change was even reported through the usual ordnance channels.
It is too bad the potential sources of all of these British uniform details are possibly sitting in as of yet untapped archives in the UK. It would be very nice to find out who all of the manufactures were as well as the suppliers of the cotton and who wove the material. Also, someone had to supply the buttons and the metal fitting, who had those contracts? I would not be surprised to learn that some of the materials that went into these uniforms were coming from North America.