Short answer: Brushes
Long Answer: You can, in GIMP/Photoshop, draw some shapes then save them (With a selection tool) as a brush. After that it's just a matter of painting them into transparent layers that show off the UVs underneath. There are other ways but this is the first that came to mind.
Edit: The texture looks like it could fit a "cartoon mode" of sorts, it's actually pretty cool.