The colour response of a camera or of the human eye is confounded by illumination. The colours recorded under yellow and blue illuminants are respectively more yellowish and bluish than they ought to be. Removing colour bias due to illumination is called colour constancy. Opinions differ as to whether colour constancy is a reasonably well solved problem or one that is still far beyond our reach. These conflicting conclusions result from the different ways that the colour constancy problem is defined. In this paper we review the color constancy problem placing special emphasis on the different definitions used. The links between the different approaches will also be made clear.