Abstract: A method of forming a coloured substrate, comprising applying to or incorporating within the substrate a diacetylene compound, and exposing the substrate to (i) a first, activating stimulus that converts the diacetylene compound from an unreactive to a reactive form, and (ii) a second stimulus that causes the reactive form of the diacetylene compound to polymerize and form the coloured substrate, wherein the diacetylene compound reverts to its unreactive form on removal of the activating stimulus and is of formula I: Y—C?C—C?C—(CH2)n-T-Q-Z (I) wherein: n=an odd integer; T=CO, CS or a bond; Q=NH, S, O, OCONH, NHCONH, NH—CHE-CONH, NHCOO or NHCSNH wherein E is H or a C1-20 alkyl group; Z=H or a hydrocarbon group containing 1 to 20 carbon atoms; Y=H or any group comprising at least one carbon atom.