This research is focused on developing a robotic painting system for artistic and graphic applications by means of an anthropomorphic robot equipped with an airbrush. Firstly, we introduce a mathematical colour spray model, based on a radially symmetric Gaussian distribution of colour intensity within the spray cone. Then, we present an experimental characterization of colour intensity in a spot, by varying the distance between airbrush and target surface and the spraying time. The experimental results of this pilot study validate the paint intensity model and provide the basis for further investigations.