1. A security device having a lenticular device comprising an array of lenticular focusing elements located over a corresponding array of pairs of image strips such that, in a first viewing direction, a first image strip from each pair is viewed by respective ones of the lenticular focusing elements and, in a second viewing direction, different from the first, a second image strip from each pair is viewed by respective ones of the lenticular focusing elements wherein one of each pair of image strips has portions defining a first image in a first colour and a second image in a second colour respectively, and the other of each pair of image strips has portions defining the first image in the second colour and the second image in the first colour respectively, whereby on tilting the device, a colour switch is observed between the first and second images.