It is sadly
not so easy. An officer?s tunic should be feldgrau (Field Grey) of course, but his trousers should be steingrau (stone grey) for most of the time of ww1. That?s one of the main problems with many modern recreations.
The reason was, the field grey uniform, introduced in 1907, faded differently, the tunics faded more quickly than the trousers. Yes, before 1914 German military officials had the time to address to such crucial issues! So the steingraue trousers were introduced in August 1914 and kept until the war?s end. Only when steingrau material was in short supply, they switched to Feldgrau again.
German officers could wear field grey trousers in WWI, if they were privately tailored. But stone grey was the regulation,
not field grey.
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This is
not an officer. But you can clearly see the difference between a field grey tunic (mod. 1907 with musician?s Schwalbennester (swallow?s nests) and stone grey trousers.