custom textures

anyone know how to make custom textures like renfurr? he made different color legacies and recolors of outfits (client sided) that work ingame and also imported a model from fortnite into dbd. live build and everything. wondering if anyone know how to do it. if anyone wants to know what im talking bout look at his twitter or twitch @Renfurr

I bet its the dev build (3.0.0, if i remember right) and afaik some of his friends has experiences of coding so they might got a chance to edit a lot of stuff, no idea since im not into game development so cant say much about it. it cant be recent build since there was no overheat for billy in his videos. i might be 10000% wrong and misleading but thats what im thinking about,