A method of forming a security device, the method comprising: providing a multilayer assembly (207, 407) comprising: a holographic image layer I (208, 308, 408, 508) having a region comprising a volume hologram that is viewable in a first observable colour, the region being formed from a material comprising a diffusible substance at a first concentration; a diffusion element (210, 410, 510) having a region that overlaps the said region of the holographic image layer and is formed from a material comprising the diffusible substance at a second concentration, different from the first concentration; and a barrier layer (209, 409) having a region that is impassable to the diffusible substance and is interposed between the said region of the holographic image layer and the said region of the diffusion element so as to preclude diffusion of the diffusible material therebetween, the said region of the barrier layer comprising a heat-transformable material, and selectively applying heat at a plurality of positions within the said region of the barrier layer, in accordance with a predetermined pattern, so as to modify the heat-transformable material such that the region of the barrier layer is selectively rendered passable (103, 229) to the diffusible substance at each of the plurality of positions, thereby permitting diffusion of the diffusible substance between the said regions of the diffusion element and the holographic image layer such that, at a plurality of positions (218, 318) within the said region of the holographic image layer and corresponding to the predetermined pattern, the volume hologram is dimensionally modified so as to become viewable in a second observable colour, different from the first observable colour. a holographic image layer I (208, 308, 408, 508) having a region comprising a volume hologram that is viewable in a first observable colour, the region being formed from a material comprising a diffusible substance at a first concentration; a diffusion element (210, 410, 510) having a region that overlaps the said region of the holographic image layer and is formed from a material comprising the diffusible substance at a second concentration, different from the first concentration; and a barrier layer (209, 409) having a region that is impassable to the diffusible substance and is interposed between the said region of the holographic image layer and the said region of the diffusion element so as to preclude diffusion of the diffusible material therebetween, the said region of the barrier layer comprising a heat-transformable material, and selectively applying heat at a plurality of positions within the said region of the barrier layer, in accordance with a predetermined pattern, so as to modify the heat-transformable material such that the region of the barrier layer is selectively rendered passable (103, 229) to the diffusible substance at each of the plurality of positions, thereby permitting diffusion of the diffusible substance between the said regions of the diffusion element and the holographic image layer such that, at a plurality of positions (218, 318) within the said region of the holographic image layer and corresponding to the predetermined pattern, the volume hologram is dimensionally modified so as to become viewable in a second observable colour, different from the first observable colour.