If you want to get rid of it all- that one paint layer- you can delete it from the world . Check out the Tutorial for info and Tips on CAWs page- https://www.thesims3.com/content/global/downloads/caw/help_en.pdf
Power User Tip:: If you later decide that you don’t really like how one of your terrain paints looks, you can choose a new file to replace the current file (which changes all instances of the old texture in your world). You can also right-click it in the Terrain Painting Tools Layer window and choose DELETE. All instances of that paint are also deleted in your world.
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