Feldgrau (not a specific color, but rather the range of grey-greens used by the German armies over the years) was a traditional uniform color scheme since the days of the German Empire, and perfectly adequate in the field by the standards of the time. It was just a natural evolution of German uniforms that had come before. There wasn't really any pressing need to break with that continuity, whether for practicality in the field or for political reasons. The Nazi regime wanted to retain a lot of the old aesthetics for political reasons; for example, Hitler is said to have rejected the prototype design for what would become the NVA's M-56 Stahlhelm on the grounds that it would break with German tradition.