A security device, comprising a transparent substrate carrying: a focusing element array defining a focal plane; and an image element array defining an array of microimages (P1,P2) against a background, located in a plane substantially coincident with the focal plane of the focusing elements; the pitches of the focusing element array and of the array of microimages and their relative locations being such that the focusing element array cooperates with the array of microimages to generate a magnified version of the microimage array due to the moire effect; wherein the image element array comprises: a patterned layer of substantially opaque material, the array of microimages being defined by the absence of the substantially opaque material and the background being defined by the presence of the substantially opaque material; and a colour layer overlapping the patterned layer of substantially opaque material and being exposed through the patterned layer where the substantially opaque material is absent, characterised in that the colour layer comprises a plurality of different optically detectable materials in a respective plurality of laterally offset zones 2. (13a,13b) of the array, the zones each being at least as large as the area occupied by one of the magnified microimages in the magnified version of the microimage array and the zones being configured such that, at a first viewing angle, the optical characteristics of a magnified microimage in the magnified version of the microimage array are determined by those of a first set of one or more of the zones of the colour layer and at a second viewing angle the optical characteristics of the same magnified microimage are determined by those of a second, different set of one or more of the zones of the colour layer such that the optical characteristics of the magnified microimage change between the first and second viewing angles. a focusing element array defining a focal plane; and an image element array defining an array of microimages (P1,P2) against a background, located in a plane substantially coincident with the focal plane of the focusing elements; the pitches of the focusing element array and of the array of microimages and their relative locations being such that the focusing element array cooperates with the array of microimages to generate a magnified version of the microimage array due to the moire effect; wherein the image element array comprises: a patterned layer of substantially opaque material, the array of microimages being defined by the absence of the substantially opaque material and the background being defined by the presence of the substantially opaque material; and a colour layer overlapping the patterned layer of substantially opaque material and being exposed through the patterned layer where the substantially opaque material is absent, characterised in that the colour layer comprises a plurality of different optically detectable materials in a respective plurality of laterally offset zones 2. (13a,13b) of the array, the zones each being at least as large as the area occupied by one of the magnified microimages in the magnified version of the microimage array and the zones being configured such that, at a first viewing angle, the optical characteristics of a magnified microimage in the magnified version of the microimage array are determined by those of a first set of one or more of the zones of the colour layer and at a second viewing angle the optical characteristics of the same magnified microimage are determined by those of a second, different set of one or more of the zones of the colour layer such that the optical characteristics of the magnified microimage change between the first and second viewing angles. a patterned layer of substantially opaque material, the array of microimages being defined by the absence of the substantially opaque material and the background being defined by the presence of the substantially opaque material; and a colour layer overlapping the patterned layer of substantially opaque material and being exposed through the patterned layer where the substantially opaque material is absent, characterised in that the colour layer comprises a plurality of different optically detectable materials in a respective plurality of laterally offset zones 2. (13a,13b) of the array, the zones each being at least as large as the area occupied by one of the magnified microimages in the magnified version of the microimage array and the zones being configured such that, at a first viewing angle, the optical characteristics of a magnified microimage in the magnified version of the microimage array are determined by those of a first set of one or more of the zones of the colour layer and at a second viewing angle the optical characteristics of the same magnified microimage are determined by those of a second, different set of one or more of the zones of the colour layer such that the optical characteristics of the magnified microimage change between the first and second viewing angles.