If you haven't previously edited whitelist.cfg, open it and add: 
"add"		"materials/materials/..."
"add"		"materials/models/..."
beneath the line that reads:
// Additional files/directories to allow



Open up pak01_dir.vpk in Notepad++ (Not normal Notepad.)
Change the -first- line (there are two similar lines.) from:

 $baseTexture "models\survivors\teenangst\zoey_color"

to:

 $baseTexture "models\survivors\teenangst\zoey_color_cob"

 

Create the following directory if it isn't made. Place the extracted files in:

steam/steamapps/common/left4dead/left4dead/materials/models/survivors/teenangst 