The present invention relates to a thermoplastic material comprising polymer, at least one charge transfer agent, wherein the charge transfer agent is substantially colourless when neutral, but which develops colour when a charge is acquired, and at least one photo acid generating agent. The present invention further relates to a method of processing thermoplastic material to form a plastic article, wherein the method comprises the step of processing the thermoplastic material at a temperature greater than the melt temperature, Tm, of the thermoplastic, wherein the thermoplastic material comprises polymer and at least one charge transfer agent, wherein the charge transfer agent is substantially colourless when neutral, but which develops colour when a charge is acquired, and at least one photo acid generating agent, and wherein the method further comprising the step of irradiating the plastic article to colour at least a region of the plastic article.