I like that there seems to be a naming scheme around colors with the key characters:
Red: The alias he took up because the color he most vividly remembered from the slaughter of his people. He also sees red whenever confronted with one of the people responsible. Might be a sly nod to the label most people he met at the time can't see past.
Yellow: Itoh's nickname gotten because the wealthy rancher's cowboys couldn't say his real name. Has the same connotations as red with people, but he also regards himself as a coward, literally yellow.
Gray: His motivations aren't to hurt or help the MC but for his own goals just like he put it "I'm not your enemy or your friend." CSA's uniform was gray too so perhaps a nod to where he's from and a part in the big tragedy in his life?
Scarlet: I believe that this means she was a survivor of Red's tribe. She'd have had to have been an infant at the time if so.
Blue: The color of the Army's uniforms at the time. Can have connotations of misery. He's real good at spreading that around.