Lights vary in their ability to render object spectral reflectances into color contrasts. When a light L1 is replaced by another L2, even if L2 matches L1 in chromaticity, systematic color shifts may occur, including a loss or gain of chromatic color. For instance, many familiar lights, when compared to daylight, dull red and green objects, rendering them closer to gray. An opponent colors method is appropriate to this discussion because it brings to the surface the notion of chromatic color, meaning actual departure from white or gray. In this article, an opponent-colors analysis leads to a matrix formulation that serves two purposes. The effects of replacing L1 by L2 are estimated with a 3x3 "rendering matrix" P. Given an object's tristimulus vector under L I, the method makes an approximate prediction of the new tristimulus vector under L2. Thanks to the opponent formulation, matrix element P22 quantifies the gain or loss of redness and greenness, while P33 expresses gain or loss of blueness and yellowness. These in fact are major effects, so the method is both quantitative and explanatory. (C) 2003 Wiley Periodicals, Inc.