The change over from green to brown paint was due partly to the availability of whatever chemical was used to make green paint "green". The worlds main supply came from the Pacific region of the world. When the Japanese overran the Pacific the supply was suddenly cut off or drasticaly reduced. Hence when you see the change over of Green vehicles to Brown in 1942/43, it was mainly due to this. An interesting side bar is that the green and brown shades on their own are near impossible to differentiate on black and white photos.
I have no idea if this plays into the change with the denims or not but it does help to answer you question Michael about the vehicle paints.