Just an aside...
WWII "Feldgrau" was NEVER an exact color. Uniforms varied widely in color, especially as cloth and dye quality went down as the war went on. Add in the fact that different uniforms are exposed to different amounts of wear, dirt, and exposure and you'll find that it is more realistic to think of Feldgrau as a range of colors.
I'm a WWII reenactor, and I have a few original WWII German uniforms. None of them are anywhere the same color as any other one.
I strongly suspect that WWI conforms to the same principle; "feldgrau" is not an exact color by any means.