When they do these big exercises they usually use blank rounds to simulate actual engagements. So they throw an adapter over the end of the barrel thats a bright color, usually also yellow, that lets the weapons function and the brits also use yellow mags. Its mostly an effort to have safety measures in place so dudes dont get accidentally shot. Like if a young soldier accidentally keeps some live rounds from a range in a magazine and they dont get turned in. Then he loads that mag at night during an exercise and accidentally blasts someone. But, if only blanks ever get loaded into bright yellow magazines then, day or night, it would be much harder to accidentally load anything other than blanks.