A method of imparting color to a plastic substrate comprising applying to the substrate, or incorporating within the substrate, a diacetylene compound of general formula (I) wherein n=1 to 20; R1=an optionally substituted C1-20 alkyl group which may contain heteroatoms; T=H, an optionally substituted C1-20 alkyl group which may contain heteroatoms or —(CH2)m—C(═O)-Q2R2; Q1=NH, CO, NHCONH, OCONH, COS, NHCSNH or NR3, wherein m, Q2 and R2 are independently selected from the same groups as n, Q1 and R1 respectively; R3 is an optionally substituted C1-20 alkyl group which may contain heteroatoms; and irradiating the substrate to impart color to the substrate is described.