I also know that the secondary colour would be red, this would be to represent the bloodiness of their raids and culture however I couldn't decide on a definite pattern. I was going to have some form of Heraldry or a Skull icon on the left shoulder. The right shoulder is going to have a scroll on it with the Marines name written on and maybe some squad markings. The fists are going to be red to represent the blood of their enemies over years of raiding, similar to the crimson fists.
In the bottom picture I considered the red reaching up the arms in a flame or blood spatter pattern as a sign of veterancy the higher it went the more senior the marine.
I have posted a picture of the three main ideas I have, however if you find any better ones, feel free to comment and give your feedback.
If you want to see the first two parts follow this link and the link at the end of that article, this is the only way we know of linking them together.