a few fellow students back in the uni days did that usually going over the UV with pencils or aquarel colors. But only crispy stuff looked good like Ink, pencil and such. Water based techniques sucked because they were blurred or cut off at UV borders and at the seams. At first thought you were aiming at camera mapping…
Someone back in the Q3 modding days did a pirate player model where they printed out the UVs and coloured it in with pencils, I seem to recall it looked quite good. I've looked for the model once or twice but I can't remember the name so I can never find it :(