How to delete texture

Question

my problem is the following: I have a 3D scan's output with color (texture) in some image file. I import this into Blender and I can see the various colors on the model. Going into the material tab, I can select the texture and see the image file pop up.

Now I would like to erase / delete part of the texture. Say there is both orange and green in my scan. I would like to only have the orange appear. Since I am pretty new to this, I tried opening the texture image file into a simple image editor and blacken the areas I did not want. Then I re-imported the 3D model with the updated image file, and I can see the changes in the viewport, but unfortunately all of the model's vertices are still considered part of the texture.

What I want is to edit the image file (or do whatever else it takes) so as to end up with some of the vertices no longer being associated with the texture image file. Is that possible at all, and if so, how ?

Any input on this porblem would be highly appreciated. If you need any further information please let me know.

Thank you very much in advance for considering my question,

Gustavo

Edit: this was part of a bigger aim and, ultimately, I have managed to achieve what I wanted. I do not think this question is relevant anymore to me.