This is what I was told and I have firm belief this is correct.
Prier to WWII in the 30?¢‚Ç¨‚Ñ¢s shoes was black in peace time. Soon as WWII hit, the shoes went to brown for the Army and Army Air Corps. I think this was the same for the USMC.
In pre war uniform the enlisted and officer visor caps had black bills the neck tie was also black and so were the shoes. About 1942 it changed to Brown. It was brown till about 1947 I believe. By that time the Army Air Force (Previously known as the Army Air Corps from WWI to 1942) became its own branch of the service. Thus, changing their uniforms from OD green or Pinks and Greens to the blue uniforms with black shoes.
I wear brown with my WWII uniforms except my Navy Blues.
Hope that helps,