After you have finished vertex shading (which is where you painted directly on the model in Edit mode like what you show), create a blank texture of whatever size you want. A power of 2 is preferred, such as 128x128, 256x256, 512x512, 1024x1024, 2048x2048 or 4096x4096. What you’ve painted on the model will be hidden by the texture, but don’t worry about that – it’s still in memory and can be seen when you view the Tool preview. Next, press Tool
Texture
Col>Txr
What you’ve painted as vertex shading will now be transferred to the texture using whatever UV mapping is applied to the model. You can now use Projection Master to continue refining the texture.
A texture created in this way will always be kind of grainy. That’s because vertex shading is limited in resolution by the number of points that the model has. More polygons will give cleaner results, but you need a very high poly count indeed to match the resolution that a texture can give.