French Uniforms
This is a re-enactment of the French uniform. However it shows how colorful the uniforms were and how un-practical they were for being used in the trenches. There was no helmet, and they were very easily spotted by the enemy. Later on in the war, the colors changed, but for most of the war, this is what they wore.
Serbia Uniforms
The standard uniform of the Serbian Army was a gray-green color. The red color was to symbolize that the men were in the infantry. Only the 1st line got this uniform. The 2nd line wore a mixture of pre-1912 colored uniforms. The old colored uniform included a dark blue tunic. The 3rd line often got no uniforms at all. They fought in their civilian clothing. The hat was the serb sajkaca, and this often was the only uniform piece that the men in the 3rd line received.
British Uniforms
British soldiers are carrying a mistletoe on December 30, 1914. Also looking at the men, (and other research) it is apparent that their uniforms are made of wool and died a Khaki color instead of red because it is harder for the opposing army to see them. Their hats/helmets weren't very protective, but they did have a sash across their chest that held bullets and other small necessities. The khaki uniforms were some sort of the picture below.
Russian Uniforms
Russian soldiers had greenish khaki uniforms. The greenish khaki color changed after washing it, so it changed color to a light brown. They wore a sheepskin hat/helmet during the winter months as well.