From 《国语·吴语》: the Wu army was described marching in ranks so vast that one side looked 「望之如荼」 (white as reed flowers) and the other 「望之如火」 (red as fire). Originally described military grandeur, now describes any vigorous activity.
mistakes
荼 is tú (reed flower), not 茶 chá (tea). Easy to misread because the characters differ by only one stroke.
