From the Model T Ford Forum:-
'The World War I Quartermaster Olive Drab was described by Charles Lemons, curator of the Patton Museum at Fort Knox, as the colour of pig slop, that is a very muddy olive brown. The colour for this Olive Drab was established by the Manual for the Quartermaster Corps, 1917, Par. 3964. The pigments used to mix Olive Drab were very simple: black and ochre.'
The whole topic:-
http://www.mtfca.com/discus/messages/708324/726541.html?1487529624