How does Crane use color to convey imagery?

In the opening paragraph of "A Mystery of Heroism," Crane's narrator describes the sky as blue, the firing of artillery as streaks of red against the sky, and the white duck trousers of the soldiers standing out as they ran and grouped together. All three colors of the American flag are evoked from the start, subtly communicating to the reader that the men in this war are all Americans—and are fighting against each other.
"Green grass" and "green woods" contrast with the gray of a shelled house and gray smoke in the air to contrast the vibrancy of nature with the waste of war. Red punctuates the narrative frequently, seen in "the crimson terror of an exploding shell" and the "flying arrows, flaming red" that Collins sees. Crane works to offer readers a strong visual sense of the chaos of a battle fought on farmland where "brown earth was being flung in monstrous handfuls."