During the Spring and Autumn Period, Duke Fu Chai of Wu led a huge army against the State of Jin. He ordered his men to form three square contingents. The middle one was dressed in white and holding white flags, which looked from a distance just like the flowers of a field full of reeds. The left unit was in red and holding red flags, which looked from afar like flaming fire all over the mountains. The right unit was in black and holding black flags, which looked from a distance like thick black clouds covering the sky. Fu Chai was trying to present to the enemy a show of overwhelming force.
This idiom describes a scene of great momentum and exuberance.