The color response of a camera or of the human eye is confounded by illumination. The colors recorded under yellow and blue illuminants are respectively more yellowish and bluish than they ought to be. Removing color bias due to illumination is called color constancy. Depending on the articles one reads, color constancy is an anywhere between a reasonably well-solved problem to one that is still far beyond our reach. These conflicting conclusions result from the different ways that the color 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.