Ch. 4

In chapter 4 Henry and his comrades are fighting and some of the soldiers are talking about some kind of officer or something and the Lieutenant gets shot in the hand. Henry isn't a very brave soldier and he often thinks of running. He shouldn't be in that war. He and his comrades are fighting in a grove and it doesn't say how the battle ends. This must be the civil war because I've heard about he blue uniforms and yellow uniforms, and I belive that that is what they wore in the civil war. And some other characters where mentioned but they are not important.
My question is: When Henry says he is going to become a blue demonstration, what does he mean I know he is talking about his uniform but I don't understand.