Abstract: A technique, comprising: providing a pixelated display unit comprising an array of pixels, wherein the optical output for each pixel is independently controllable; applying a first colour filter overlay to a viewing surface of said pixelated display unit, said first colour filter overlay defining a first colour pattern including one or more areas of a single colour continuously extending over a plurality of pixels; operating said pixelated display unit to produce regions of differing brightnesses within said first colour pattern; peeling said first colour filter overlay from said viewing surface of said display unit; applying a second colour filter overlay to said viewing surface of said display unit, said second colour filter overlay defining a second colour pattern including one or more areas of a single colour continuously extending over a plurality of pixels; and operating said pixelated display unit to produce regions of differing brightnesses within said second colour pattern.