CEF (Canadian Expeditionary Force) soldiers wore coloured, rectangular patches on their sleeves, starting in about 1916. The colour of the patch depended on the division: red for 1st Division, blue for 2nd Division, French Grey for the 3rd Division, green for 4th Division.
There were additional distinctions for individual battalions. There's a pretty good summary here:
http://canadiansoldiers.com/insignia/formations/cefform.htmThe gentlemen in the photo shown are all officers - they look happy enough to pose in front of the gun, but none of those folks would dirty their hands by actually firing that gun!