How to hex Skin for models.

I’m wondering how to hex skins (not models)
Example: Nick (normal white suit)
Nick (modified black skin suit)

How do I have them at the same time
At Gmod?

You have to extract the contents of Left 4 Dead 2 using GCFscape and then convert the textures to an editable format and then edit the textures properly, bumpmaps and so on, and then recompile it in Valve format. Package the model and its new textures and put them in your Garry’s Mod folder.