Green and gold! Aussie marines FTW!!!
I like what you've done. Pretty cool back story. I've been thinking up a scenario for a while about a kill team being sent after a rumored STC, and a marine from this chapter would make a cool addition to it.
My only criticisim, (and it's a very minor one,) is that the codex does allow for different designations for rank and veteran status. There are plenty of chapters who use different ways to show this stuff. Like stripes and/or different colours on helmets, knee pads and other places on their armour. So it's not really a divergence. It's still in the codex. They just use something different than what the Ultramarines use. Keep in mind that the codex has a large section in it about uniform and also about camoflage. The Ultras just use one small part of it. (I recomend you read Insignium Astartes to get a bit more info on it.)
And it's good to see a skill tree that has both Lore skills and combat skills in it rather than just one or the other. I'm really tempted to give one of these guys a go one day.
I'm pretty sure my spelling is wrong in a couple of places, but I dont care because i'm sick.