A cylindrical coordinate colour space (lightness, saturation/chroma, hue) is derived from an opponent colour space in the RGB space. It is shown how cylindrical coordinate colour models widely used in the literature are related to or can be reduced to the derived model, thereby contributing to creating a unified cylindrical coordinate colour model. In particular, the widely used saturation expression max (R , G , B) - min (R , G , B) is derived from the proposed model. Properties of the derived chroma and saturation expressions are examined. Finally, some applications of cylindrical coordinate colour spaces are briefly reviewed.