It is your normal accouterments and gear of members of the Military Police Corps or others assigned as guards in Service A for more formal occasions.

The white allows for high visibility for the men, who beyond simply controlling POW's and Brigs had many other duties in the rear areas. They directed and controlled traffic, handled civilian crimes or complaints against service members, secured or retrieved materials of war, controlled movement of people with or without passes and prevented desertion or AWOL service members and guarded against infiltration by enemy forces. While most of their uniforms were identical to men in other arms of the service, but when visibility, or smartness was called for the white leather and linings could come out. So say like here for use as guards during court proceedings.

Nor has this gone out of style, while the US Army has moved away from white leather belts and linings, internationally you still find it, and even for ceremonial detachments, color guards, and guard functions as befitting for the US Navy. And the USMC's dress uniform notably includes a white leather belt for the enlisted version.

What we should note though is that the men in these pictures were not actually from the MP Corps themselves, but were drawn from across the First Infantry Division(then consisting of the 16th, 18th, and 26th Infantry Regiments, and even other units as men were transferred in and out as units left Europe for home. However as they were on guard duty they got the white equipment, including the web belt(still the M1936 IIRC) in white, white leggings, gloves, and helmet covers. While the 1ID's MP Battalion(the 793rd) did provide guards and men for many of the functions. The Internal Security Detachment at least on the day of the picture you posted is mostly men from infantry units. One special designation then can be seen on the side of their helmets, it is the special insignia for the IMT(international Military Tribunal).

Here is a wonderful picture in color closer up and from the side so you can see more of the uniform.