A previous paper reported the construction of a tooth colour atlas. This was used to carry out an extensive measurement and classification of tooth colour. The distribution data obtained showed that 80 percent of tooth colour was concentrated in a central hue band of tooth colour space. Utilising the "blending effect" of composite resin restorations the colour samples could be restricted to this narrow band. A modification of the tooth colour atlas producing a simplified nine colour guide for composite resins was proposed and theoretically this new colour system would achieve approximately ninety-five percent undetectable colour matching. This paper explains the development of the colour guide and reports on two clinical trials to determine the validity of the theoretical conclusions.