Here are a couple of shots I took of Belgian Force Publique uniforms in the Musée Royal de l’Armée et d’Histoire Militaire in Brussels. They appear to be made of (quite heavy) wool not any sort of lighter cotton. Can't have been much fun in the jungle!
The one thing I noticed was how dark blue the uniforms were compared with the original painted miniatures on the Foundry website, which inspired me to buy them