Even those who dream in color are likely to imagine World War I in black and white. The vast majority of the images that have come down to us, both photographs and a few movies, are monochromatic. Even All Quiet on the Western Front, the classic film about World War I that was released in 1930 is available only in black and white (having apparently escaped the colorization efforts of Turner Classic Movies).
There is, however, a trove of color photographs from World War I available online. The photographs were taken by the French army during the last two years of the war using a color film that had been developed by Auguste and Louis Lumiere in 1907. Here's one showing French troops posing in a trench.
One of the points that comes through in even a very cursory look at the photographs is the extent to which the war was in fact a world war. There are soldiers pictured from various French colonies including Senegal, Algeria, and Indochina. Woodrow Wilson's call for national self-determination must have been very unnerving to allies fighting the war with considerable help from their colonies.