Depends what you want out of your zombies; dried and dessicated, waterbloated, freshly bruised and wounded, bloodless and pale, skinless?
A layer of palid wych flesh for deathly pale, rakarth flesh a more sickly tone or cadian flesh for much more living is a good start. Another option is to go for pure white, and wash it with dilutted flesh tone until you get the colour you're happiest with.
Once you have a base skin colour you're happy with apply a wash into the recesses of whatever you wish to tone it with; purple for bruising, green for more traditional zombie, sepia or fleshtone for a fresher look, or red wash and then a layer of tamiya clear red witha drop of purple ink for skinless. I'm certain somone with more experience will be along soon to point you in the right direction, these are just the notes i've taken from reading guides here.