Here is how I painted the undead skin on my Plaguebearers of Nurgle. This method is a slightly modified version of Mengal Miniatures' "Zombie Rotting Skin" guide.

1. Paint the whole mini in Corax White . This is an off-white colour that adds to the skin tone. For bonus points do a zenithal highlight of Dead White (or another pure white) with an airbrush.
2. Wash all of the skin and wounds with a 1:1 mix of Lahmian Medium and Reikland Fleshshade .
3. Wash all of the skin with a 1:1 mix of Lahmian Medium and Athonian Camoshade , twice. Make sure that the wash is fully dried before going over it again.
4. Add Carroburg Crimson into all of the wounds. I also painted the large patches of organs with Screamer Pink at this point. The rest of the organs will be fully painted separately, but I like to take advantage of any overspill from red and purple washes.
5. Glaze around all of the wounds, boils and bloated areas with a 1:1 mix of Lahmian Medium and Carroburg Crimson .
6. Glaze particularly sore and distended looking areas with a 1:1 mix of Lahmian Medium and Druchii Violet . I like to go into deep folds, under ribs and the underside of limbs with this mix too. The idea being to make it look like blood pooling internally.
7. Highlight raised areas of the skin that are still looking pretty green with Ionrach Skin . Then add a finer highlight of Pallid Wych Flesh to the same area. At this point I also highlight boils, inside wounds and edge highlight split skin as well.
8. 1:1 mix of Lahmian Medium and Carroburg Crimson again. The idea here is to add a red tinge to the fresh highlights in the wounds again.