Abstract: A composition comprising a plurality of discrete carrier-swellable polymer particles (preferably polyNIPAM particles) and a corresponding carrier (e.g. water), which particles have a low polydispersity index and are present in an amount of at least 0.1% by weight of the composition may be used to impart structural-image properties (such as structural colour) to a substrate by coating or printing methods. Additional benefits of adherence to low-energy surface substrates and enhanced rheological properties for printing compositions may also be provided. The compositions and methods used in the invention allow visual effects or security applications to be incorporated into substrates in a low-cost and convenient manner.