Shipboard: Standard red-security, with a division specific badge instead of the standard delta.
Why would you want the 'marines' to wear the same uniform color that the security officers wear? They are not the same area of specialty.
My thinking? Shipboard, the Marines would be part of the standard security forces. Unless their was a boarding action, they'd not really have much to do in between missions.
You never know. They could spend the time training, for example.
I just disagree with the notion that SF Marines would simply be drawn from existing ships' crews (i.e. redshirts). That would not be realistic at all, IMHO. More specialization would be required. Being a soldier is a full-time job unto itself.