3 references (but you really need to read it all) to yellow zinc chromate primer in BODY CHANGES MATRIX MB & GPW Darryl Deppe's Time line
Olive Drab. Usually Red primer. However, some MB's dated Feb '42 have been found with yellow zinc chromate primer on the tub and front fenders. ( Doug. Ingram 2014)
Please Note! Some Very Early Slat Grill MBs have been found with Yellow Zinc Chromate Primer
Primer Used on VEP Slat Grill ( First few Months of Production) Black Primer or Combination of Black Primer and Yellow Zinc Chromate
Source:-
http://jeepdraw.com/PART_COLOURS.html