Death Guard – Tutorial: Step-by-step Terminator
I imagine the Death Guard still wearing basically their original armors with the original pre-heresy colors on them. Only that since they embraced Nurgle, the stuff got really dirty, rusty and rotting. In addition, rather than demonic and mutatic Marines with all kind of weird extremeties, I imagine the Death Guard in a cold, industrial look. So that they look a little like rotten versions of their loyal colleagues.
The resulting color scheme is like a mix of the pre-heresy and the classical greenish Death Guard as shown by GW. So much for the background.