A specifically-synthesized water-soluble photochromic acid dye has been applied by screen printing on to silk, wool and nylon. The influence of processing parameters and the nature of the substrate on the photochromic response of the printed materials has been evaluated using specifically-developed colour measurement methodology. The printed substrates show photochromic colour change from weakly coloured to an intense blue colour under sunlight or UV irradiation. In most cases, the photochromic colour build up is higher on wet substrates than after drying. Computer aided molecular modelling techniques have been used to explain aspects of the photochromic behaviour.