There was in fact color photography during WW1. The Germans and French did produce some color photos at the time. The process was in the developmental stages for the most part. This is not to be confused with hand colorized images of the period that were very popular. Color photos much rarer than B&W images of course.

100-Year-Old Color Photos Show a Rare Polychromatic Glimpse of World War I
War photographers documented life on the frontlines, and these photos were taken with actual color film 100 years ago.

Hans Hildenbrand: The German front in rare color photos, 1914-1918 - Rare Historical Photos
These incredible color photographs of the German battlefront during the First World War, 1914-1918, were taken by Hans Hildenbrand.
