As a once half-serious amateur photographer with a bit of an interest in photographic history, I can confirm that colour photography in various processes was well established long before WWI, although they didn’t necessarily produce accurate or lasting colours.
There is no shortage of colour photographs from the WWI era, but there is also a lot of colour printed images of the postcard type which weren’t straight reproductions of original colour photographs but of photographs which were artificially coloured for the printing process.
The Lumiere brothers developed the Autochrome process in France in 1903 (I know this because I could remember that the Lumieres invented one of the early and effective colour processes, and Google provided the rest ), so there is nothing surprising about colour photographs of French troops in WWI. Kodak (USA) and Agfa (Germany) also produced effective colour films by the start of WWI, so both sides had access to colour film in WWI.
Anyway, one thing on Google led to another, so here’s some more WWI colour photos. http://www.worldwaronecolorphotos.com/
And here’s a mix of photos and the printed postcard type of colour image I mentioned. It’s not hard to pick the difference. The Black Watch field kit is impressive. http://completeall.com/History/WW1-in-Color.html