After The War of The Beast, the Imperium needed more marines. Thus the Halo Brethren were amongst the many chapters founded during the Fourth Founding. The chapter lost a whole company when they, along with the Sable Swords and Imperial Fists, assaulted the Eversor Temple on Terra. Despite being a part of such an important event, these mooks have no canonical color scheme or history beyond that, leaving it up to you to fill in the blanks.
Contrary to popular belief, they are not spartans, and most certainly do not have any members named John; instead, those guys are all in another chapter.