There is loads of useful information in "British Forces Motorcycles" By Orchard and Madden. It's really worth searching out a copy on the internet.
But to answer your question briefly, the following is a guideline. The standard pre war finish of parade ground Bronze green (or RAF blue). Was not suitable. So at the start of the war the WO directive called for a mat Khaki Green. Around 1942-43, the colour was changed to Earth Brown, and in 1944 the colour was again changed to Olive Drab. Of course there were no hard and fast rules as older colour paint stocks were used up or mixed with newer colours. Ron
PS does anyone know the BS paint code number for Earth Brown. I fancy doing my WD/L this colour. I have some BS 499 which I've always known as Service Brown?