Abstract: The present invention relates to thermoplastic material comprising polymer and at least one polychromic substance, wherein the polychromic substance is a functionalised diacetylene having the formula which has the general structure: X—C?C—C?C—Y—(CO)n-QZ wherein X is H or alkyl, Y is a divalent alkylene group, Q is O, S or NR, R is H or alkyl, and Z is alkyl, and n is 0 or 1. 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 of the thermoplastic, wherein the thermoplastic material comprises polymer and at least one polychromic substance as defined above; and further comprising the step of irradiating the plastic article to colour at least a region of the plastic article.