What i was saying is usually when you modify an existing texture it's as follow:

• you open your original texture in a paint program (Photoshop, Painter, Gimp...)

• you modify it by the way you want (using purchased psd or png files, etc..)

• save it with another name not to overwrite your original files

• in Poser, you call your modified texture maps instead of the original ones for all the body parts you want the modify

• as Antonia said above, you can save it as a material collection

• and if Poser was open when you did the modify, click on "reload textures" in the render tab on the menu bar at the top of your Poser window (if you can't see the changes immediatly, try to change the room in Poser, it seems you can view the changes only when changing room)

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