In WW1 uniforms lagged a bit behind the times! In the days of massed musket fire, those bright colors were needed to distinguish groups of friends from foes in a battlefield covered in smoke from black powder. By WW1, though, misidentification had become less of a problem, smokeless powder was in use, and by the end of the war, they'd all switched to nice, neutral, inconspicuous colors! These here are colonial troops, so it might be that they're trying to avoid friendly fire, or it might just be for the picture.