Recoloring specific part of a model

Hello, I am new into modelling and therefore have to ask, how do I recolor specific parts of the model? Like, take the HL2 Citizen, I want to recolor the suit only, how do I do that?

Get VTFEdit, find the citizen_sheet file in the VPK, extract it and edit the file in Paint.NET or Photoshop.

Can I like, make a gmod addon with it, which adds new Citizen models?
So like, I dont have to recompile a complete new model, with the edited sheet, and make it as addon, but use the existing one with the edited sheet
Also, in the VPK is cititzen_sheet, and citizen_sheet_normal, the citizen_sheet is only a vmt, the _normal is the vtf, but the vtf is already in bumpmap style, how shall I recolor it?

Just use SetSubMaterial on the edited material.