Prior to the mid-22nd century, dermal art was very common on Denobula. Phlox's parents insisted he studied it, which came in usefull when he used his skills to create faux prison markings on Degra's arm. (ENT: "Stratagem")
In the prequel comic Star Trek: Countdown, it was explained that it was a tradition on Romulus that when a loved one died, those who suffered a loss would paint their grief upon their skin. In time the paint would fade, and with it the period of mourning. Nero and his crew, however, burned their tattoos deep so that they would never fade.