I mentioned the Boeing Images site while asking William about the gas tanks on the Stearman today.
You can find it here: http://www.boeingimages.com/
William: I found this photo: http://www.boeingimages.com/archive/Stearman-Kaydet-Trainers,-Model-75s,-at-Stearman's-Wichita,-KS-Facility-2F3XC5CN98H.html
It seems to show production planes, during or before the war years (based on the factory size, the number of aircraft, etc.), all with painted fuel tanks.
This photo can be dated (http://91.189.44.144/vintage/userFiles/nationalities_stearman_001.jpg (http://91.189.44.144/vintage/userFiles/nationalities_stearman_001.jpg)) as April 1942, and shows the Chinese, Canadian, Peruvian, US Army, and USN from above: http://www.boeingimages.com/archive/Stearman-Kaydet-PT-17-Primary-Trainers-built-for-Peru,-Great-Britain,-China,-US-Navy,-and-US-Army-2F3XC5BH8LU.html
So far, I've not seen a period photo that seems to show an unpainted tank? Maybe they were left unpainted when production shifted to all-silver schemes?
At any rate, don't miss these two pics of the parts aircraft stacked like firewood!
http://www.boeingimages.com/archive/Stearman-PT-Trainer-Parts-in-Storage-2F3XC5YM5QV.html
http://www.boeingimages.com/archive/Stearman-PT-Trainer-Wings-in-Storage-2F3XC5YMHTG.html