The CIECAM97s type of colour appearance models results in some changes in hue and saturation for)r series of colours of constant chromaticity, but changing luminance factor. To keep hue and saturation constant for such series, a model in which the dynamic cone response is based on a power function has been developed. By optimizing its parameters, its correlates of hue, lightness, colourfulness, brightness, and saturation perform nearly as well as those of CIECAM97s. A similar performance is achieved in a modified power model, which is more physiologically plausible. (C) 2003 Wiley Periodicals, Inc. Col Res Appl, 28, 82-88, 2003; Published online in Wiley InterScience.