Not only is there blood spilt in battle as soldiers are wounded and dying in The Red Badge of Courage, but as the title suggests, red is symbolic of much more. The wound that Henry receives is red, certainly, but ironically it is no badge of courage; rather it is a wound from the butt of a crazed soldier as Henry is running away.  Thus, it is a supreme irony as is the battle in which Henry's red-blooded American passion drives him to his act of bravery which becomes merely an exercise in futility since his regiment retreats.  So, red comes to symbolize human sacrifice that is of no value in a futile war against his countrymen, a war not worth the spillage of man's most precious fluid.

In the final chapter, Henry reflects upon the recent past,

He had dwelt in a land of strange, squalling upheavals and had come forth. He had been where there was red of blood and black of passion, and he was escaped. His first thoughts were given to rejoicings at this fact.

However, the haunting vision of his former cowardice and the man who helped him though his own red blood spilled covers the "purple and gold" of glory for Henry as his memory intervenes. Nevertheless, Henry is able to put "the sin at a distance" and  

He felt a quiet manhood, non–assertive but of sturdy and strong blood. He knew that he would no more quail before his guides wherever they should point. He had been to touch the great death, and found that, after all, it was but the great death. He was a man.

Henry loses the blood of a boy with his head wound earlier, but later acquires the blood of a man who has truly been battle-tested. Red, then becomes significant in yet another way.

Given that Stephen Crane's novel, The Red Badge of Courage, is about war, the use of the color red to symbolize the blood spilled is poignant. War is violent. War is deadly. Soldiers must be courageous and willing to fight to the death. With war, many are killed, spilling their blood upon the ground. Crane wishes to show the realities of war in the novel. The use of blood, and red, defines the realities of the bloodiness of war.

That said, some soldiers are hurt during war. Their injuries prove to be their badge of courage (a metaphor), showing that they were willing to face the horror and reality of war. The "red badge of courage" illustrates the blood spilt courageously by soldiers.