Brush Painting Shade

Hi everyone!
I'm trying to do a painting app. Really simple. I have gameobjects and i access to their texture, duplicate it, paint it in the UV position where i touched and replace the tetxure with the new one.
The problem is the setpixel. The app is working but seems to be not smooth.
I was wondering if there is a way to create a shader or an alternative to setpixel. I dont know, perhaps a shader or with GC.

Thanks a lot!