The paper deals with metamerism, which is the term applied to cases in which colors match under one set of viewing and illuminating conditions but fail to match under a different set of conditions due to differences in their spectral reflectance distributions. Conservators often have to consider the potential for metamerism. The author's goal is to determine if it is possible to predict when a mismatch will occur and if there are tools available to the art conservator to aid in reducing metamerism. Potential metamerism may be minimized through colorimetry, which encompasses the physics and psychology of color and its quantification. Illuminant metamerism and observer metamerism are described, and it is shown that colorimetry can quantify the problem. Controlling metamerism involves matching the eye's visual responses for all the illuminants of interest. Trial and error and computer-colorant formulation are two approaches to this goal. Computer-colorant formulation is probably preferable because of the time saving. The paper concludes with a call for systematic exploration of the metamerism problem so that the techniques illustrated can be applied to historical paintings by the conservation community.