Paul, that is the $64,000.00 question. There is no hard fast rule to stock color as when the new batch of stock oil was put in the tank the stocks would come out lighter in color and as the dirt, grime and whatever was introduced to the oil the stocks would darken. Also, how was the stock treated after it left the factory? "Hey Joe, my stock is drying out." "Hey, the only thing I have is this old used motor oil." "That'll work." You get my point. Also, there will be a difference how center or core wood takes color as opposed to an outer cut because grains are different between the two.
I have an IBM carbine with a walnut stock that just will not darken anymore. The grain is very tight and just won't take it.
A dark reddish brown is a good start though.