Sounds like you're doing well so far. What colour have you painted the Bolters? I would suggest you paint the back pack in the same colour. Leadbelcher is a good colour for the bolters and back packs, it's a dark silver metallic colour. Agrax Earthshade would be a good colour to shade it too.
If you need to highlight the armour a bit, try adding a little bit of Nurgling green to the Deathworld forest and give it a light dry brush. A purple shade wash (can't remember what it's called) around the gribbly bits works well on Plague Marines.