Here's My (Alphonsusjude) Article on How to quickly paint Plague Marines:
Step 1: Prime the Miniature Chaos Black
Step 2: Base Coat the Miniature in Catachan Green
Step 3: Heavily Dry-brush Scorched Brown
Step 4: Heavily Dry-brush Vermin Brown
Step 5: Dry-brush Camo Green a little lighter than the Scorched or Vermin brown
Step 6: Lightly Dry-brush Bleached Bone
Step 7: Paint the bolter, and any tubing Boltgun Metal
Step 8: Paint the edging of the armour Dwarf Bronze
Step 9: Wash The armor edging and with a Black wash
Step 10: Wash the Entire model and bolter (minus armor edging) with a brown wash
Step 11: Highlight Skulls and other areas with bleached bone
Step 12: Wash Highlighted areas lightly with a brown wash
Step 13: Finally Paint the eyes Red.
(also in this step I did a light purple wash in particular areas. Just a personal preference)
Hope this helped!