Commercial photochromic dyes change rapidly and reversibly from colourless to coloured when activated by ultraviolet irradiation. This paper contributes towards the growing interest in the development of chromic textiles for creative design, functional or smart purposes. The optimisation of a methodology to evaluate the colouristic properties of photochromic textiles using traditional ultraviolet illumination source and colour measurement equipment is described. The experimental factors requiring careful control, including temperature and the time interval between ultraviolet irradiation and measurement, are identified. On the basis of a statistical evaluation of validity, a consistent, reproducible semi-quantitative methodology is described, which is appropriate for evaluation of the performance of commercial photochromic dyes applied to textiles, a detailed investigation of which will form the basis of subsequent papers in this series.