The Colour is called Field Gray, or Feldgrau, and is really a greyish green. Tamya field grey is fairly close, but a little grey and a littel light, Humbrol slate grey is just about spot on.
The colours changed during the course of the war, as the quality of the uniforms decreased. In 1939 - 41 the uniforms were very well made of hi quality wool, but towards the later half of the war the germans inroduced increasingly larger quantities of synthetic viscous rayon which was not so hard wearing.
A general rule of thumb is to paint early war uniforms greener, and late war uniforms more grey, but of course there are always many exceptions to this.
To complicate matters further, it was pretty common for officers to have their uniforms made up by a private tailor, so this introduced even more variations.