Hi there and welcome to the forum.
German combat uniforms in the early stages of the war ( 1939 - 1940 ) consisted of a field grey blouse with dark blue collar and stone grey trousers. This was soon discontinued and replaced with field grey cloth. Tamiya XF-65 - Field Grey has no equivalent in Humbrol. I do not have a Revell color chart so I can not give you an exact color reference. Field grey has a slight green tinge to it and appears more green than grey. As the war progressed, material became scarce and the uniforms by 1944 tended to be more grey than green and the insignia was mouse grey on a field grey backing. Before 1943 all belts and leather equipment were brown in color. After 1943 the leather equipment was black in color.
The USA entered the war in December 1941 after the attack on Pearl Harbour and the first US soldiers in Northern Ireland word olive drab ( Humbrol 155 ) uniforms with the M1918 helmet ( similar to the British soupdish helmet ), leggings were canvas and webbing and pouches sand colored. The US retained the Olive Drab uniforms for the duration of the war in Europe. The helmets changed before 1943 to the familiar M1 steel helmet. By 1944 the M1941 combat jacket in olive drab with olive drab trousers were common and in general use.
I hope this helps.