QUICK ANSWER AT 1:10 Create a new texture and change its alpha value to 0, use the alpha as the factor of a mix node with this new texture on input 1 and the rest of your nodes on input 2. --------------------- Ive been extensively using the mix node in Blender but couldnt figure out how to simply add paint on top of my node setup without having to blend it or do some weird trick. The answer was actually so simple... hopefully it will help someone else! You should also experiment with the different blending mode of the Mix node for adding shadows and highlights! Instead of using an Alpha you can use a mask (a black and white texture, handpainted or from a noise node from example) as a factor to tell Blender where you want each of the two image texture to be applied. You can keep adding mix nodes after mix nodes to create some crazy textures, mixing procedural, handpainted, and image textures together seamlessly! And then baking all that to export to your game engine. If you have any question please leave a comment and Ill try to help.
How to texture paint in Blender 2.8 with layers just like in Photoshop