I've noticed that alot of pre-herset Imperial fists armies have the red helmets with the white stripe down the center. Why is that? Is it a marine status (veterans) or honor marking? Any story or fluff behind it? Are all Imperial fists helmets that way?
Post-heresy its a common indicator for a Veteran Sergeant, drawn from Guilliman's Codex. Its popular among pre-heresy IF armies due to the fact the pre-heresy marine pictured in the IF Index Astartes article has the red helmet/white stripe combo. Its not universal among the Fists though, as the IF in Collected Visions invariably have yellow helmets. You could use it to represent sergeants or veterans if you like, or ignore it altogether (after all, the Fists aren't like the Ultras, and will have a certain amount of variation between different companies).