...Wojtku, looking at your pic in post #16, I think the lighter elevator colour is exhaust staining from the engines.
The enlarged shot of the questioned pic reveals that the colour of the elevator was the same used for the rudder. However it can be clearly noticed that the exhaust staining there runs across the stabilizer and the elevator causing the smudge in the middle of the tail surfaces.
Here is another shot of a such marking I found in the Flugzeug Profile no.18. The caption says He-111 over Balkans. In the shot you may notice the elevators painted either yellow or white. I would say ,the yellow. Also the engine cowlings were of the colour. Please notice the lack of the white band on the fuselage. No wonder the yellow engine cowlings and rudders were for Balkans area while the white band for the Mediterranean one.