Working with Textures, Models, and 2D - Mastering Unity Scripting - page 318

Create a new material using the TextureBlender shader and assign the background
texture to the MainTexture slot, as shown here:
Creating a new material from our TextureBlender shader
Add a new Quad , Plane , or Mesh to the scene and remove its collider, if any.
This object will receive the final painted output, although the click detection
will occur on a duplicate mesh. I am keeping the final output mesh and the
click-detection mesh separate to allow the destination mesh to have other
collider types or components if required.