There are extant examples of both types of graffitti (scratched and painted). In a sight in Turkey there are scratched drawings similar to yours in what was possibly the ready area of gladitorial training camps or smaller arena. Pompei too has examples of both scratched and painted graffitti with some of it being very descriptive about the sexual exploits of a certain individual. (No doubt the influence for some of the material in the Rome series.) The painted grafitti was done with any number of "organic paints" i.e. woad, madder, lamp black, soot etc.
In the Castillo in St. Augustine there are several murals or graffitti work done by soldiers garrisoned there in the 18thc. that depict french ships, english ships, the names of soldiers and sailors and dates. These are scratched into the stucco that covers the coquina rock in the garrison rooms and what was the prisoner's rooms.
Think of it this way...like today, gaffitti artists don't like to hang around to get caught defacing property, so they will do it on the fly/run with what ever is handy. If its somewhere where the artist/author has time to kill (jail, ready room, barracks, garrison room) there is a greater likely hood that he will want his mark permanent and has time to mess with it. Thus the scratching of graffitti.