A method of manufacturing a security device, the method comprising: providing a substrate having a viewing region; providing an obscuring layer (11,12) in the viewing region; providing a first image being a colour-composite image formed of first and second sub-images respectively with first and second sets of colour components of respective colours, each of the first and second sub-images having an assigned side of the obscuring layer, wherein each of the first and second sub-images is not homogeneous; providing a mask representative of a second image, the mask being indicative of locations of colour components to be swapped to the side opposite their assigned side of the obscuring layer; applying the mask to the colour-composite image by swapping colour components of the first and second sub-images at each location indicated by the mask to the side opposite their assigned side, to form: - a mask pattern (3,5) representing the swapped colour components on the swapped side, and - a background pattern (4,6) representing the unswapped colour components on their assigned side; printing the mask and background patterns on their corresponding side of the obscuring layer; wherein the obscuring layer between the printed patterns reduces the visibility of colours on the one side when the other side of the obscuring layer is viewed in reflection, the obscuring layer allowing light to pass through the viewing region when the security device is viewed in transmitted light, whereby - when either side of the viewing region is viewed in reflected light, the patterns on that side are dominantly visible and may be distinguished at least by their colours; and - when the viewing region is viewed in transmitted light from either side of the viewing region, the viewing region is sufficiently transparent that colour mixing between the overlapped different colours of the overlapped patterns results in the colour-composite image being visible. providing a substrate having a viewing region; providing an obscuring layer (11,12) in the viewing region; providing a first image being a colour-composite image formed of first and second sub-images respectively with first and second sets of colour components of respective colours, each of the first and second sub-images having an assigned side of the obscuring layer, wherein each of the first and second sub-images is not homogeneous; providing a mask representative of a second image, the mask being indicative of locations of colour components to be swapped to the side opposite their assigned side of the obscuring layer; applying the mask to the colour-composite image by swapping colour components of the first and second sub-images at each location indicated by the mask to the side opposite their assigned side, to form: - a mask pattern (3,5) representing the swapped colour components on the swapped side, and - a background pattern (4,6) representing the unswapped colour components on their assigned side; - a mask pattern (3,5) representing the swapped colour components on the swapped side, and - a background pattern (4,6) representing the unswapped colour components on their assigned side; printing the mask and background patterns on their corresponding side of the obscuring layer; wherein the obscuring layer between the printed patterns reduces the visibility of colours on the one side when the other side of the obscuring layer is viewed in reflection, the obscuring layer allowing light to pass through the viewing region when the security device is viewed in transmitted light, whereby - when either side of the viewing region is viewed in reflected light, the patterns on that side are dominantly visible and may be distinguished at least by their colours; and - when the viewing region is viewed in transmitted light from either side of the viewing region, the viewing region is sufficiently transparent that colour mixing between the overlapped different colours of the overlapped patterns results in the colour-composite image being visible. - when either side of the viewing region is viewed in reflected light, the patterns on that side are dominantly visible and may be distinguished at least by their colours; and - when the viewing region is viewed in transmitted light from either side of the viewing region, the viewing region is sufficiently transparent that colour mixing between the overlapped different colours of the overlapped patterns results in the colour-composite image being visible.