For an unlimited time, even you can have undead skin in four easy stages! Start with a good rotting flesh undercoat. Yum.
Next, grab some leviathan purple wash (alternately, for skin that has been dead longer use a blue wash). Only wash the upper areas of your skin...
The oddly named thrakka green wash is next. Since you didn't wash your armpits, under your chin, and other shady places, these areas will get green.
Get your rotting flesh back out and do a highlight on the raised areas like bony protrusions and muscles.