Well, it looks like I'm a bit late to the modding game as far as Space Marine is concerened, but I've been messing around with some helmets for Titus and pals.
I stuck Sedonus in a Master Crafted helmet with some laurels added for a little bling, while I put Leandros back in the standard helmet you see him in at the beginning of the game. What I'd really like to do is replace that with the damaged model you see in a later cut-scene, but I can't seem to find the game assets for that. If there's anyone out there who's run across these, please let me know.
For Titus, I used the Salamander's Champion helmet, colored in the Ultramarines livery. I can't decide which verion I prefer, though...
Gold face plate
If anyone's interested in any of these textures, just let me know.
For those who are on the green side of the ork blood debate, I found the texture files for the gore, and tried altering the color. The end result, though, was that when the gore map was applied to the armor, not only was the gore green, but the entire armor set turned green as well. I was wondering if anyone out there with a bit more experience than I might have some thoughts on how to get around having the armor look like it was dipped in ork after a melee kill.