Working with Textures, Models, and 2D - Mastering Unity Scripting - page 320

In addition, add the TexturePainter script to the object and assign the Brush
Texture field to the brush texture itself and the Dest Mat ield to the Custom_Mat
material from the Project panel, as shown here:
Creating a click detection quad
Now, run the application and start clicking on your mesh. When you do, paint
strokes will be applied to the texture and display on the viewport, as shown in
the following screenshot:
Texture painting completed