So something like this?
The method to get that result is:
- Start from black
- Heavily drybrush brown over the whole model
- Apply 5-6 successively lighter drybrushes of brown mixed with an increasing amount of off-white (use light grey for a cold colour, sand for a warmer one), up to a final drybrush of pure off-white
- Add some splotchy washes of brown, green, purple and red all over the armour to give it that patina of grime, and for added Nurgle, sparingly use a bit of Typhus Corrosion/Ryza Rust around joints and seams
Done!
That's actually a pretty quick method, shouldn't take more than about 15 minutes to get the armour up to the washing stage, then it's just adding those and filling out the rest of the details.
Oh, and welcome to Dakka!